What is the single most influential book every mathematician should read? If you could go back in time and tell yourself to read a specific book at the beginning of your career as a mathematician, which book would it be?

There are so many, and I've already seen three that I would mention.  Two more of interest to lay readers:
The Man Who Knew Infinity by Robert Kanigel.  Excellently written, ultimately a tragedy, but a real source of inspiration.
Goedel's Proof by Nagel & Newman.  Really, a beautiful and short exposition of the nature of proof, non-euclidean geometry, and the thinking that led Goedel to his magnificent proof.
