Reference requests are common. However, I wish to make this a list of books which every undergraduate,majoring in mathematics should own or at least read.It may be a bit subjective.

Please include 1 book per answer .If there is any additional(or alternate) recommendation by you or some other person for the same topic please include it in the comments.

I am flagging it for community wiki.

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Cider with Rosie, by Laurie Lee. I read this during my undergrad, and found it rather enjoyable! –  user1729 Oct 30 '12 at 12:21
See mathoverflow.net/questions/761/undergraduate-level-math-books Also, real analysis courses must always use Rudin! –  wj32 Oct 30 '12 at 12:21
@user1729 : ) ${}{}{}$ –  Matt N. Oct 30 '12 at 12:22
I don't think such books exist. –  Michael Greinecker Oct 30 '12 at 12:23
Every undergraduate should own the course catalog. –  Gerry Myerson Oct 30 '12 at 12:27