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Community wiki? –  Akhil Mathew Jul 21 '10 at 20:23
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I think script usually refers to the text of a play or movie. Perhaps you meant manuscript? –  François G. Dorais Jul 21 '10 at 20:24
    
Ahhh, thank you, fixed. (The German word for "lecture notes" is "Skript", that got me confused). –  Hendrik Brummermann Jul 21 '10 at 20:51
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I can give you an "anti-recommendation": don't get Cameron's "Sets, Logic and Categories". While it's a nice short introduction to some set theory and logic, the final chapter on category theory is too short and not at all well explained. It is however a neat little book for logic and sets... –  Seamus Aug 3 '10 at 13:11
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Best with respect to what metric...and for whom? This is a very fuzzy question. –  Pete L. Clark Feb 13 '11 at 7:03
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17 Answers 17

up vote 21 down vote accepted

Lang's Algebra contains a lot of introductory material on categories, which is really nice since it's done with constant motivation from algebra (e.g. coproducts are introduced right before the free product of groups is discussed).

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Lang's algebra also has significant typos that make it frustrating to read if you do not have enough mathematical maturity. –  user126 Jul 24 '10 at 11:07
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re the typos: this is one instance where buying a used book that someone has marked up can be a really good idea. –  Isaac Jul 24 '10 at 16:08
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Bergman has a HUGE (ca 200 pages) companion to Lang's Algebra book math.berkeley.edu/~gbergman/.C.to.L . But even with this companion, I am not really sure if Lang is a good textbook to learn from. It does the right things, but it often does them in sloppy and/or subtly wrong way. I wouldn't say it does very much category theory either. –  darij grinberg Dec 31 '12 at 21:51
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I once heard from a rather well-known professor that he and another professor didn't quite get why a proof of a theorem was "obvious". It turned out that it wasn't so obvious, and they wrote an article about the proof. Quite fun. I think Lang has an absolutely fantastic idea of what topics to include, but the exposition is not ideal. –  Dedalus Jun 7 '13 at 10:47
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Categories for the Working mathematician by Mac Lane

Categories and Sheaves by Kashiwara and Schapira

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pfft, only punks read Cat & Sheaves. These lecture notes by Schapira are better: people.math.jussieu.fr/~schapira/lectnotes/AlTo.pdf They use some nice specific examples and problems to develop category theory. With the added bonus of learning some nice alg top along with it. :D:D:D –  BBischof Jul 21 '10 at 23:19
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Uh-why do only "punks" read Cat & Sheaves? I happen to think it's a terrific and modern introduction to homological algebra and related fields. –  Mathemagician1234 Sep 13 '11 at 18:35
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What do you mean by punks? What's wrong with the book. Your comment really made me not want to read the book (especially because it has 7 upvotes, although it doesn't explain anything. –  Bananarama Mar 10 '13 at 17:26
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And when you get bored of reading, let the Catsters take over. (78 videos on Category theory!)

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Another book that is more elementary, not requiring any algebraic topology for motivation, and formulating the basics through a question and answer approach is:

Conceptual Mathematics

An added benefit is that it is written by an expert!

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Could someone edit this to give the link a name, so people immediately know what is being linked to? It's Lawvere and Schanuel's Conceptual Mathematics. Which is a really wonderful book for learning some category theory if you don't have the background to understand the heavy duty algebraic topology etc examples that enter into some discussions of CT at a very early stage. –  Seamus Aug 3 '10 at 13:07
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@Seamus Ok done! –  BBischof Aug 3 '10 at 15:06
    
Are you implying that usually books are not written by experts? :-? –  Andrea Ferretti Aug 22 '10 at 13:24
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@Andrea hehe no, but Lawvere is particularly great! –  BBischof Aug 22 '10 at 18:37
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One particular prolific author was rumoured to write (and publish) many of his books in order for him to learn their subjects :-) –  Robin Chapman Aug 23 '10 at 18:35
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Awodey's new book, while pricey, is a really pleasant read and a good tour of Category Theory from a logician's perspective all the way up to topos theory, with a more up to date view on categories than Mac Lane.

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Peter Smith has criticised Awodey's book for being pitched too high to be an introduction to categories logicmatters.net/2008/06/awodeys-category-theory-ch-1 –  Seamus Aug 5 '10 at 10:37
    
I will admit to having mainly used Awodey as source material while putting together my own introduction to categories lecture course material. As such, it was highly pleasant - but I am not a good example of what a newcomer'll need... haskell.org/haskellwiki/User:Michiexile/MATH198 –  Mikael Vejdemo-Johansson Aug 5 '10 at 11:44
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+1 for Awodey,which is the only book I would consider to teach category theory to undergraduates. –  Mathemagician1234 Sep 13 '11 at 18:39
    
I wanted to add that his book is now available in paperback at half the price of the hardcover edition: Amazon –  bwkaplan May 30 '12 at 18:06
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The nLab is a great resource for category theory.

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I take the OP to be asking about introdutions to category theory. nLab is not a good introductory text... –  Seamus Aug 4 '10 at 7:30
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@Seamus You're right. But it's a good general reference, in the same way that wikipedia is a good general reference but shouldn't be used as a text. –  Kevin H. Lin May 4 '13 at 16:03
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I'm also a fan of Tom Leinster's lecture notes, available on his webpage here. In difficulty level, I would say these are harder than Conceptual Mathematics but easier than Categories and Sheaves, and at a similar level as Categories for the Working Mathematician.

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I discovered those notes recently and in my opinion they are great! –  Pandora Dec 6 '11 at 21:32
    
He also has a book coming out: Basic Category Theory. –  J W 2 days ago
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I've read a fair amount of Sets for Mathematics and found it to be a gentle introduction.

http://www.amazon.com/Sets-Mathematics-F-William-Lawvere/dp/0521010608/ref=pd_sim_b_5

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The first few chapters of Goldblatt's Topoi: the categorial analysis of logic provide another fairly elementary introduction to the basics of category theory.

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Goldblatt's book (which is very beautifully written, by the way) is available online in its entirety here. –  Hans Lundmark Aug 23 '10 at 6:51
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I struggled for years to understand category theory until I met Goldblatt's book; then the struggle was over. –  MJD Mar 11 at 0:23
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Paolo Aluffi, Algebra: Chapter 0 has category theory woven all through it, particularly in Chapter IX of course. I can tell that randomly sampled pieces of the text are well-written, although I have never systematically read longer parts of it.

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I have gone through this book very carefully. It is indeed an excellent algebra book, but the last chapter is not very good. –  Hui Yu Apr 30 '13 at 6:49
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Arbib, Arrows, Structures, and Functors: The Categorical Imperative

More elementary than MacLane.

I don't know very much about this, but some stripes of computer scientist have taken an interest in category theory recently, and there are lecture notes floating around with that orientation.

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as a young student, I enjoyed peter freyd's fun little book on abelian categories. The nice thing about Freyd's book is it isn't boring, and it has little pieces of wisdom (opinion) such as the remark that categories are not really important, you just define them so you can define functors. And in fact you just define functors so you can define natural transformations, the really interesting things.

Of course you may disagree, but blunt debatable assertions (like this one) always make for more interesting reading. Another provocative remark by this author is the observation that he himself seldom learnt math by reading books, but rather by talking to people.

From the nice link above I learned that Goldblatt also quotes a remark (which may have inspired Freyd's) by Eilenberg and Maclane that categories are entirely secondary to functors and natural transformations, on page 194 where he introduces these latter concepts.

Leinster's notes linked by Patrick, look nice - a bit like an introduction to Maclane's Categories for the working mathematician, chatty and full of debatable assertions, (many of which I disagree with, but enjoy thinking about). He does not give much credit, but I believe the adjoint functor theorems he quotes without proof, (GAFT,...) may be due to Freyd. Leinster's notes are easy reading and informative.

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Eilenberg and Mac Lane's original paper: General theory of natural equivalences says that they defined "category" to define "functor", and "functor" to define "natural transformation". But I get the impression that the category theorists of today don't take that remark all that seriously. –  Uday Reddy Dec 7 '13 at 18:44
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First Chapter of Jacobson's Basic Algebra -II.

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MATH 4135/5135: Introduction to Category Theory by Peter Selinger (17pp). Concise course outline. Only wish it covered more topics. Available in PS or PDF format.
http://www.mscs.dal.ca/~selinger/4135/

Handbook of Categorical Algebra (Encyclopedia of Mathematics and its Applications) by Francis Borceux. Rigorous. Comprehensive. This is NOT free, but you can see the contents/index/excerpts at the publisher's web site, listed below. This is a three volume set:
(v. 1) Basic Category Theory, 364pp. (ISBN-13: 9780521441780)
http://www.cambridge.org/catalogue/catalogue.asp?isbn=9780521441780
(v. 2) Categories and Structures, 464pp. (ISBN-13: 9780521441797)
http://www.cambridge.org/catalogue/catalogue.asp?isbn=9780521441797
(v. 3) Sheaf Theory, 544pp. (ISBN-13: 9780521441803)
http://www.cambridge.org/catalogue/catalogue.asp?isbn=9780521441803

Reprints in Theory and Applications of Categories (TAC). This site has 18 books and articles on category theory in PDF, including several by F.W. Lawvere.
http://www.tac.mta.ca/tac/reprints/index.html

Abstract and Concrete Categories-The Joy of Cats by Jirı Adamek, Horst Herrlich, and George E. Strecker (524pp). Free PDF. Published under the GNU Free Documentation License. Mentioned already by Seamus in reference to Wikipedia's external links for Category Theory, but worth repeating, because it's very readable.
http://katmat.math.uni-bremen.de/acc

A Gentle Introduction to Category Theory (the calculational approach) by Maarten M. Fokkinga (80pp).
http://wwwhome.cs.utwente.nl/~fokkinga/mmf92b.html

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Wikipedia has some nice free texts linked at the bottom. There's an online version of Abstract and Concrete Categories, for example.

Steve Awodey has some lecture notes available online too. (Awodey's newish book is expensive, but probably rather good)

Patrick Schultz's answer, and BBischoff's comment on an earlier answer also have good links to freely available resources.

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Barr and Wells, in addition to Toposes, Triples and Theories, have written Category Theory for the Computing Sciences, a comprehensive tome which goes through most of the interesting aspects of category theory with a constant explicit drive to relate everything to computer science whenever possible.

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Lawvere, Rosebrugh. Sets for Mathematics.

Pierce B. C. Basic category theory for computer scientists.

José L. Fiadeiro. Categories for Software Engineering.

Martini. Elements of Basic Category Theory.

Burstall, Rydeheard. Computational category theory. Requires ML background.

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