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 contain a ton of interesting-looking math but mostly sit unread.Tim Gowers' Princeton Companion to Mathematics comes to mind, but I don't own it and I'm not sure if it's breadth-over-quality.