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.

When I was about 12 years old, I read Euclid. The way the theorems were arranged, the way Euclid had proved the theorems were very beautiful to me. I didn't know that something so beautiful could exist in the world. 
Burton's book Elementary Number Theory was the next book that I have regarded as a beautiful one. The books that I have read about number theory before that were very neat and rigorous in their treatment of topic but Burton's book not only treated the subject thoroughly but also gave historical introduction before each chapter. That was what made it a artist's work! I thoroughly enjoyed reading the book. 
Landau's Foundations of Analysis is also a very inspiring book.
