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.

W. W. Sawyer's 'Prelude to Mathematics' is a great book that really opened my eyes. It can be read almost without any knowledge of mathematics.
I also believe that any mathematician ought to read 'Flatland'. It is a beautiful story it gave me my fist real intuition about higher dimensions.
