Spivak is an excellent choice but may be too advanced depending on his level. I would also strongly recommend any of the books in the Art of Problem Solving ser

Spivak is an excellent choice but may be too advanced depending on his level. I would also strongly recommend any of the books in the Art of Problem Solving series: https://artofproblemsolving.com/store/list/aops-curriculum https://artofproblemsolving.com/store/list/aops-curriculum I've got a PhD in bioengineering but I'm currently going through Introduction to Counting and Probability and I'm really enjoying it. Some others (not AOPS series): Nelsen - Proofs Without Words Polya - How to Solve it Strogatz - Nonlinear Dynamics and Chaos