Fun but serious mathematics books to gift advanced undergraduates. I am looking for fun, interesting mathematics textbooks which would make good studious holiday gifts for advanced mathematics undergraduates or beginning graduate students.  They should be serious but also readable.
In particular, I am looking for readable books on more obscure topics not covered in a standard undergraduate curriculum which students may not have previously heard of or thought to study.
Some examples of suggestions I've liked so far:


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*On Numbers and Games, by John Conway.

*Groups, Graphs and Trees: An Introduction to the Geometry of Infinite Groups, by John Meier.

*Ramsey Theory on the Integers, by Bruce Landman.


I am not looking for pop math books, Gödel, Escher, Bach, or anything of that nature.
I am also not looking for books on 'core' subjects unless the content is restricted to a subdiscipline which is not commonly studied by undergrads  (e.g., Finite Group Theory by Isaacs would be good, but Abstract Algebra by Dummit and Foote would not).
 A: I recommend Paul Halmos' Automathography for an account of an extremely interesting life in mathematics. It's a joy to read!
A: In addition to the other choices, maybe you want to think about some other types of books.
You might want to check out many of the books by Clifford Pickover.
Additionally, you might want to go through this large list of fascinating mathematical fiction books and titles and you might find interesting choices.
Have fun!
A: Euler's Gem is a great book, you should check it out!
A: Ronald Brown's Topology And Groupoids gives a highly original and unusual first course in topology through basic category theory and the fundamental groupoid instead of the fundamental group. This allows Brown to present homotopy constructions in a very geometric way and to exclude homology altogether. 
A: I personally did not read this, but a friend of mine read A History of Abstract Algebra by Israel Kleiner for a term paper he was writing.  It was real good, especially the parts about Noether and Dedekind. From what I gather all the information came from this book.  I plan to buy it soon myself.
A: *

*Check into Generatingfunctionology by Herbert Wilf. From the linked (author's) site, the second edition is available for downloading as a pdf. There is also a link to the third edition, available for purchase. It's a very helpful, useful, readable, fun, (and short!) book that a student could conceivably cover over winter break.





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*Another promising book by John Conway (et. al.) is The Symmetries of Things, which may very well be of interest to students.





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*One additional suggestion, as it is a classic well worth being placed on any serious student's bookshelf: How to Solve It by Georg Polya.

A: For a fun read, which has the additional advantage of dividing into independent chapters which can be consumed in bite-sized chunks over the holiday season, how about

Proofs from The Book, by Martin Aigner and Günter Ziegler

And +1 to the OP's initial suggestion of Conway's On Numbers and Games.
A: Modern Graph theory by Bela Bollobas counts as fun if they're interested in doing exercises which can be approached by clever intuitive arguments; it's packed full of them.
A: Fifty challenging problems in probability. Here is a larger list. Hope it helps
A: I would gift "Visual Complex Analysis" by Needham. It is a very beautiful book with a deep geometric intuition about complex numbers that is not typically covered in an undergraduate complex analysis course.
I would also gift Stillwell's "Roads to Infinity: The Mathematics of Truth and Proof", one of the most beautiful treatment of the infinity concept that I encountered and that does not stay at the undergraduate level.
A: Saunders MacLane, Mathematics Form and Function. I'm reading it right now. It gives a wonderful birds eye view of (undergraduate) mathematics. The book is mostly self-contained for undergraduates and upwards, but it certainly helps to know a lot of math already.
A: Off the top, no particular order:


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*Conceptual Mathematics - Lawvere and Schanuel

*Sets for Mathematics - Lawvere and Rosebrugh

*A Walk Through Combinatorics - Bona

*Combinatorial Species and Tree-Like Structures - Bergeron, Labelle & Leroux

*Ordinary Differential Equations - Arnold

*What Are and What's the Purpose of Numbers - Dedekind 

*Collected Works of Karl Menger - Menger  

*Algebraic Number Theory and Fermat's Last Theorem - Stewart


Just a couple if you're interested in applied areas:


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*Theory of Gambling and Statistical Logic - Epstein

*Theoretical Introduction to Programming - Mills

*Elements of Statistical Learning - Hastie, Tibshirani & Friedman

A: Topics in Algebra by I.N.Herstein.
A: I would suggest Graphs and their uses by Oystein Ore.
One can find there some game theory, shortest route problems, coloring maps on surfaces, etc. It is very pleasant to read.
A: I should like to add In Pursuit of the Traveling Salesman by William Cook. 
A: I'd recommend the new Dover edition of Michael Barnsley's Fractals Everywhere.  Barnsley is an expert on iterated function systems (IFS) and shows how fractal geometry can be used to model real world objects.  Subjects include metric spaces, dynamical systems, fractal dimension, fractal interpolation, the Julia and Mandelbrot sets, and measures on fractals.  The style is engaging and the book is well illustrated.
A: I just got a hold on the book Primes of the Form $p=x^2+ny^2$ by David A. Cox and I think it has the required features. 
In this book the author manages to present advanced algebraic number theory via historical point of view. He starts with the works of Fermat, Euler, and Gauss, and finishes with class field theory and complex multiplication. 
I would certainly recommend this book as a fun and interesting book for an advance undergrad or beginning grad student (actually to anyone who likes number theory).
A: Julian Havil’s Gamma: Exploring Euler’s Constant is a very readable introduction to a number of topics tied together by connections with the Euler-Mascheroni constant $\gamma$, topics that in my experience seldom get more than a mention in an undergraduate curriculum. It’s not a textbook, but it’s not a popularization in the usual sense; call it a popularization for younger mathematicians.
Graham, Knuth, & Patashnik, Concrete Mathematics, is one of the most readable textbooks I’ve seen; some of the material in it may be covered in undergraduate courses in combinatorics or probability, but much of it will be unfamiliar.
A: The Cartoon Guide to Calculus is an amazing book for calculus beginners and does satisfy all of the OP's needs.
A: This is a great book on the art of inequalities. Lots of problems inside as well:
Cauchy-Schwarz Inequality
Also available online in PDF form here:
Cauchy-Schwarz PDF
A: The Sensual (Quadratic) Form, by John Conway.
I confess that I've only read the first chapter, but what I've read seems to fit the bill perfectly: Easily readable essays on an interesting topic that students don't normally see. Each chapter is independent from the others, and even many number theorists I know haven't heard of "topographs," the unique approach to visualizing quadratic forms that Conway develops in the first chapter.
EDIT: I want to add Proofs that Really Count by Art Benjamin and Jenny Quinn. Again, haven't read it myself, but I've met both the authors and seen them give (excellent) talks, and I've never heard anything but praise for this book.
A: As someone in your target audience, I recommend A=B, by Marko Petkovsek, Herbert Wilf and Doron Zeilberger (with a foreword by Donald E. Knuth).
It's basically a book on generating combinatorial identities programmatically, inspired by Exercise 1.2.6.63 in Knuth's Art of Computer Programming, Volume 1:

[50] Develop computer programs for simplifying sums that involve
  binomial coeffcients.

The mathematics is not above an undergraduate's head, and a lot of the results are intuitive and attractive.
Best of all, the book is available in its entirety from the website (although paper copies can also be purchased)
A: Surreal Numbers by Knuth.
A Novel which turns into pure mathematics.
A: Here are some fun books on geometric topology:


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*The Shape of Space by Jeff Weeks

*The Knot Book by Colin Adams

*The Wild World of 4-Manifolds by Alexandru Scorpan


Scorpan's book is much more difficult than the first two, but it's still suitable for many beginning graduate students.
Disclaimer: While I'm currently reading Scorpan's book, I haven't actually read either Weeks' or Adams' book.  These are titles that I know from reputation and would eventually like to read myself.
A: I would highly suggest Matrix Groups for Undergraduates by Kristopher Tapp. This is extremely readable--you feel like you are doing little more than reviewing some linear algebra and analysis, but BAM you realize that you've just had an extremely gentle, but useful introduction to the basics of Lie groups.
A: I propose Counterexamples in Topology and Visual Group Theory.
Les nombres remarquables by François le Lionnais is also interesting, but it is written in French; I do not know if there is an equivalent in English, maybe The Penguin Dictionary of Curious and Interesting Numbers.
A: Here are some of my favorite books in this category.
Real Infinite Series
A Radical Approach to Real Analysis
Counterexamples in Analysis
Galois Theory for Beginners
All of these are great for pedagogical purposes. They are all well written and accessible to any undergrad. The first one, even calculus students can appreciate. The next two teach and supplement any analysis course. The fourth one gives an excellent history and an introduction to Galois theory, how/why it began, why the quintic is impossible to solve in radicals, how Gauss made an algebraic connection to geometry and provided a construction of a regular 17-gon, and how three of the ancient geometric problems (doubling a cube, squaring a circle, and trisecting an angle) were proved impossible once and for all.
A: Two of my favorites are written by John Derbyshire. They both combine a historical narrative with mathematical discourse with plenty of charts and illustrations to help visualize concepts:


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*Prime Obsession: Bernhard Riemann and the Greatest Unsolved Problem in Mathematics (Plume Books, 2003) ISBN 0-452-28525-9

*Unknown Quantity: A Real And Imaginary History of Algebra (Joseph Henry Press, 2006) ISBN 0-309-09657-X
A: I've not read it myself, but a friend told me he really enjoyed Robertson and Webb's Cake-Cutting Algorithms: Be Fair If You Can, which seems to be both mathematically serious and readable, and moreover is on a topic not usually covered in the undergraduate curriculum.
A: Look at Numbers, by Ebbinghaus and 7 co-authors. It has nice discussions about the real and complex numbers (aimed at mathematicians, not neophytes), and also the  quaternions, octonions, p-adic numbers, and infinitesimals.
A: Imre Lakatos, Proofs and refutations: The logic of mathematical  discovery
It's a joy to read and everybody will  learn something new from it, even your math professor (if he didn't already read it).
A: I suggest Information Theory, Inference and Learning Algorithms by David MacKay. It is a book about Information Theory and codes, topics of great practical importance which aren't well covered in undergraduate courses. It has many exercises, some of which are very challenging. Opening it at random is bound to reveal something interesting, such as how the Bletchley Park codebreakers worked, what's the difference between British and American crosswords, and why we don't reproduce asexually. It requires a minimum of prior knowledge and is also a great introduction to probability and Bayesian statistics.
A: Here are several books that I have looked at frequently. 
Proofs and Confirmations, David Bressoud
Winning Ways For your Mathematical Plays Vols. 1 to 4, Berlekamp, Conway, Guy
Integer Partitions, Andrews and Eriksson
Number Theory in Science and Communication, Schroeder
Fractals, Chaos, and Power Laws, Schroeder
The first part of The Road to Reality, Penrose contains a primer on the math required in modern physics.
A: A Gentle Introduction to Art of Mathematics by Joseph Fields is really nice.
A: I enjoyed this one:


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*Galois Theory by Ian Stewart

A: The Book of Numbers by John H. Conway & Richard Guy. Although this does fall into the "popular mathematics" arena, it contains breadth and depth that will be of interest. Whilst I read many formal textbooks on mathematics, The Book of Numbers is a book I often return to when working on a problem. It contains many gems.
A: As leery as I am of adding yet another book (co)authored by John Conway to the list, I have to plug the amazing The Symmetries Of Things, a tremendous introduction to the basics of symmetry groups and tilings.  There are things it could be better at (the fact that there's no connection with root lattices and Dynkin diagrams is a little odd to me), but it's still a fine introduction that does take some deep dives into questions of group theory.
A: I'm a bit surprised that no one has yet recommended Galois' Dream: Group Theory and Differential Equations by Michio Kuga. 
The way that Kuga connects different parts of mathematics in this book is just amazing and eye-opening for one early in their studies, and the free and elegant way he connects ideas in Galois theory to topology and differential equations is just inspiring. It can feel like a "pop math" book, but it is certainly deep and rigorous enough.
I am fortunate enough to be able to read it the book in the original Japanese, and the language is also rather colorful; I don't know how much of the colorfulness is lost in translation.
A: I was given An Illustrated Theory of Numbers by Martin H. Weissman as a student. The visual approach to the subject is really immersive and the book is very well written and covers a broader range of topics than one would likely find in an undergraduate course while still being highly accessible.
A: Dissections: Plane & Fancy by Frederickson, and the second side of the same coin: The Banach--Tarski Paradox by Tomkowicz and Wagon.
