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.

The two pages in the 1960 Compton's Encyclopedia article on "calculus": the left page was essentially on derivatives, the right page on integrals. A few excellent pictures. It showed a few of the many amazing things one can do with calculus, and made it appear obvious, simple, unburdensome. Two pages, simple ideas with huge potential. Wonderful. Yes, mathematics is powerful and magical.
(In contrast, any book that makes calculus difficult is inimical. Long books on calculus? Terrible. Too bad calculus has become a filter/weeder subject. Perverse.)
