Some reference for crystal bases:

Intro course on crystal bases

Most refered to is book by Hong and Kang, and another one by Bump and Schilling (focusing on combinatorics).

Here is a course: Quantum groups, combinatorics and geometry seminar

Daniel Bump’s website on crystals

Crystal for Dummies

Connection of crystals to number theory(?):

Here is a course: Combinatorial models in Representation Theory