I liked "Algorithms Unlocked" by Cormen (https://mitpress.mit.edu/books/algorithms-unlocked). It's the same author that co-wrote "Introduction to Algorithms" (or CLRS) that's referenced in a few other comments, but this one focuses on the what and why of algorithms and pulls samples from a few areas of interest.

It's not too long (~200 pages), so might be worth checking out before diving into one of the more comprehensive textbooks.

If you're interested in 3D programming, I found "3D Math Primer for Graphics and Game Development" to be the most accessible for the math fundamentals. I just discovered it's now available freely online as well: https://gamemath.com/.