What was the book that opened your mind to the beauty of mathematics? Of course, I am generalising here. It may have been a teacher, a theorem, self pursuit, discussions with family / friends / colleagues, etc. that opened your mind to the beauty of mathematics. But this question is specifically about which books inspired you.
For me, Euler, master of us all is right up there. I am interested in which books have inspired other people.

Apostol's Introduction to Analytic Number Theory. It's a beautifully written and self contained book. Even if you cannot solve all the problems, just reading the text will take you a long way. One of the best number theory books I've seen. 
