Before everyone starts posting their Amazon affiliate links, look at the requirements that makes the LessWrong thread so valuable:

  2.  You must have read at least two other textbooks on 
      that same subject.
  3.  You must briefly name the other books you've read on
      the subject and explain why you think your chosen
      textbook is superior to them.

  Rules #2 and #3 are to protect against recommending a bad
  book that only seems impressive because it's the only
  book you've read on the subject.