For curiosity: Understanding Digital Signal Processing, by Richard G Lyons. I read it when I was starting college as an undeclared major, and I honestly didn't understand half of it, but I was getting into music production and it taught me the ins and outs of impulse responses. I later declared a major in Applied Computational Math primarily because of that book.
For intellectual ability: it wasn't a particular book, but it was an honors-level Real Analysis class that forced me to re-learn the way I learn mathematics.
For intellectual ability: it wasn't a particular book, but it was an honors-level Real Analysis class that forced me to re-learn the way I learn mathematics.