[feklar]

Where to begin depends on what your goal is, mine was to be able to complete Sussman's SICM book (and later, his Functional Differential Geometry book).

The book I used to do this was Advanced Calculus by Loomis & Sternberg because it covers classical mechanics, potential theory, differentiable (Banach) manifolds, differential equations, (multi)linear algebra, fundamental theorum of calculus and the Fourier transform. The exercises are not very difficult compared to a lot of other texts (Spivak's Calculus) so this is an accessible math book as I don't have any formal math training.

I also liked the Mathematical Preliminaries crash course in the Art of Programming Vol 1 because it led me to looking into probability which has turned out to be an infinite rabbit hole of discovery. (a grad students highly opinionated list) https://www.amazon.com/gp/richpub/listmania/fullview/1F85VWN...