spivaks calc on manifolds also formed, as a full time teacher in 1971 or so, the basis for my understanding of advanced calc, but it was courant's diff calc vol1 that lifted me as a college freshman in 1960, into the higher level of math once and for all, after a first glimpse in high school from allendoefer and oakleys "principles of mathematics".
I also like Mumfords first three preliminary chapters ("redbook") on algebraic geometry, and Beauville's Complex Algebraic Surfaces.
but mostly i only began to really learn math at an advanced age (about 32) when i was already a little beyond the level of textbooks and more into learning from professors. I read a lot of textbooks by Widder, Lang, Rudin, Royden, Halmos, Zariski, Herstein, Artin, Munroe, Spanier, Ahlfors, Kamke, Hausdorff, Hilbert, ... preparing for research, but I didn't understand them. Grad school at Brandeis and later Utah, was eye - opening.
So I never really learned anything from a textbook that even approximates what I got from live teachers like Herb Clemens, David Mumford, Phillip Griffiths, Boris Moishezon, Hugo Rossi, Robert Seeley, Ron Stern, Maurice Auslander, Alan Mayer, Bernard Teissier, Arnaud Beauville, Robert Varley, Pete Bousfield, Ed Brown Jr., Johnny Wahl, Mike Schlessinger, Lynn Loomis, David Kazhdan, Frans Oort, Mike Artin, Robin Hartshorne, John Tate, Raoul Bott, Mike Spivak, Paul Monsky, Ken Chakiris, Bob Friedman, Dave Morrison, Ron Donagi, Barry Mazur, Janos Kolla'r, Miles Reid, Heisuke Hironaka, and many others (Enrico, Maurizio, Fabrizio, Fabio, Sevin, Gerald, ...).
A human being who understands the subject can often tell you more in one sentence than you can get from reading a whole chapter of a book.
