I know someone who wanted to build up a solid mathematical foundation after years of considering themself to be bad at it, and ended up loving Spivak's book on

I know someone who wanted to build up a solid mathematical foundation after years of considering themself to be bad at it, and ended up loving Spivak's book on calculus. (Another well-written book on calculus, in a very different style, is the classic book "Calculus Made Easy" - this book is old enough to be available on Project Gutenberg.) You can learn linear algebra without learning calculus, but to really care about linear algebra you have to know why it's useful. The main lesson from (multivariable) calculus is that you should approximate functions by linear functions whenever possible - so the basis of almost everything else in math is a deep understanding of linear functions (and the occasional quadratic when you need that little bit of extra oomph). Once you manage to convince yourself to care about linear functions, the subject magically becomes easier to learn. I don't have any especially great textbook recommendations - perhaps Strang's book?