Specifically for the GRE the books I used was Manhattan series both the course and the 5 lb. book of problems. For learning in general the ones that have helped me the most are Ultralearning as mentioned, Tim Ferris' series of books, Art of Learning by Josh Waitzkin. I also rate Atomic Habits by James Clear and The War of Art by Steven Pressfield to create the underlying habits that are needed for this.

As a bit of a left field set of books, I love getting inspired by amazing fantasy characters. Characters that have superpowers. Kvothe from the King Killer Chronicles, Luo Ji in the The Dark Forest. I always walk away inspired when I read a scene about a character rapidly acquiring skills.