For me the book that changed my perspective on proofs was : Proofs from THE BOOK by Martin Aigner and Günter M. Ziegler. I always thought that proofs are just means to an end, but this book showed me that proofs can be more interesting than the end result. The book begins with six proofs of the infinitude of the primes, and you can read it in any order. It contains some of the most elegant proofs of mathematical theorems.