TITLE: Most inspirational mathematical books
QUESTION [5 upvotes]: I would like to know which books on mathematics (from university texts to divulgative pop-math books) inspired you the most. 
My choice is Spivak's Calculus, which is, IMHO one of the most inspirational mathematics books I've ever read: it has very meaningful examples and clear explanations.

REPLY [3 votes]: The Princeton companion to mathematics is surely a tour de force. It is a joint effort of many leading mathematicians, and show the beauty of mathematics rigorously in an inspiring way.