Context: I'm a Grade 11 IBDP student and very fond of math. Assuming that I get the spare time to self-study material alongside the syllabus, I wish to spend it constructively (the only step I've taken thus far in learning mathematics is to complete HS math courses on Khan Academy up to and including Integral Calculus). I want to tackle a heavy-duty calculus book like Spivak or Apostol before entering college (if you feel aspiring mathematicians in HS would be better served by other objectives, please share them in your answer).
I decided against compiling a list of textbooks from various sources and followed the advice given in this answer on Physics Forums (an arbitrary choice; I just thought it was a well-written answer), which mentioned the SMSG textbooks. I am, however, concerned about their obsolescence.