Go ahead and get your free copy of Avi Wigderson's Mathematics and Computation https://www.math.ias.edu/avi/book.

Broadly speaking there are two types of "CS Theory":

1. "What is computation and what can be computed?" That's what Wigderson's book is about. Complexity theory.

2. "What's the best way to compute something?" covered by "Introduction to Algorithms" and the like -- sth people ask on interview questions. In addition to the references here, http://people.cs.uchicago.edu/~laci/17discrete/ gives some good puzzles to work through.