Interesting math books for children Are there any interesting books on math for children?  Let's break this into two questions: interesting books on math for children in elementary school and interesting books on math for children in middle school.  
The only book I can think of that might work for kids is The Little Schemer by Friedman and Felleisen.  To me, that seems almost as much a math book as a programming book.  
And there's also Alice’s Adventures in Wonderland and Through the Looking Glass, by Lewis Carroll, which, although they don't really have any explicit math content in them, do have the sort of contradictory, paradoxical type of humor that many math people like.  
Also Raymond Smullyan's books, but I think those might appeal mainly to older teens, but I'm not really sure.
Any ideas?
 A: 
And there's also Alice’s Adventures in Wonderland and Through the
  Looking Glass, by Lewis Carroll, which, although they don't really
  have any explicit math content in them, do have the sort of
  contradictory, paradoxical type of humor that many math people like (not the Pillow Problems, these are just problems, for older kids).

Actually Lewis Carroll does have more mathematically inclined children books like "A Tangled Tale" and "Pillow Problems". And they share the humor we like (not Pillow Problems, these are just problems, for older children).
Especially I liked his own symbols for trigonometric functions. Detexify doesn't seem to be able to find there LaTeX versions, so I've made an image:

As Lewis Carroll explains, this notion came from the "old trigonometry", where sines and cosines were real lines. Seems it was not only Feynman who invented his own notion for them. I've read both books in Russian, where they were accompanied with his letters to children and his textbook "Symbolic Logic". That was indeed a nice collection, I failed to find an English equivalent, but sure it should be somewhere.
A: I just found this book on Amazon: "Math Puzzles and Games, Grades 6-8: Over 300 Reproducible Puzzles that Teach Math and Problem Solving" by Terry Stickels.
It has a blurb by Martin Gardner on the back, so it's probably quite good.
Also, Flatland.  How could I forget Flatland?
A: Here is a link
http://mathcircle.berkeley.edu/
with lots to offer (not only) kids. There is a further link to a book section. Plus you might find a comparable endeavor near you, if it's of interest.
A: I have not read it, but you could try "The Number Devil: A Mathematical Adventure", by Hans Magnus Enzensberger.
A: I pored through this at least a couple times from 2nd to 5th grade:

Lots of illustrations; it looks at recreational-type math puzzles using a loose narrative structure with a cast of characters. It gave me some basic exposure to concepts in number theory (amicable numbers, perfect numbers), geometry (polygonal numbers, Möbius strips), logic (classic puzzles, the barber paradox), and combinatorics (permutations, combinations). And they don't say so, but they do skirt around the Collatz conjecture, those jerks.
Use Amazon's "look inside" feature to make sure it's your style! This book is largely expository, rather than problem-solving, but it's likely to get kids to look at a few concepts they haven't encountered before.
