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.

I suspect this topic will turn out to be simply a list of great books in mathematics - and that's a beautiful thing!
For me, the one that really did it was Raymond Smullyan's "What Is the Name of This Book?" which I was given when I was about ten years old. That's the book that really made me see the beauty and the sheer joy of mathematical logic, and I have no doubt that it led me, through winding paths, to many of the other titles already mentioned. (Including GEB, Proofs From the Book, Byrne's Euclid, and many, many others) 
As a close second, I would nominate Gardner's "Aha!", which deals with intuition in solving mathematical puzzles. I had this book at a similar time, and I'm sure it affected my thinking deeply, just as Smullyan's did. 
