The Textbook That Unleashed Ramanujan's Genius
I'd love to see recommendations of similarly "powerful" math books in circulation today.  And I mean books that actually taught you a ton of math, not books that <i>contain</i> a ton of interesting-looking math but mostly sit unread.<p>Tim Gowers' <i>Princeton Companion to Mathematics</i> comes to mind, but I don't own it and I'm not sure if it's breadth-over-quality.