Martin Aigner, Günter M. Ziegler — Proofs from THE BOOK — 3rd ed., Springer, 2004, 240 pp. ISBN 3-540-40460-0
Paul Erdős always refered to proofs that were particularly elegant or powerful as proofs from The Book, a book with a transfinite number of pages, held by God (in which he didn’t believe), containing all the most beautiful proofs of all mathematical theorems.
Aigner and Ziegler present about sixty proofs that could have been from The Book. Some are understandable by someone with a moderate mathematical culture, but others do require advanced notions and may be rather hard to fully understand (unless you’re a real mathematician, which I am not). Nevertheless, it makes a very interesting read.