
It is about the beautiful segment of mathematics,namely,combinatorics,which teaches how to reduce an apparently (ugly)cumbersome problem of counting to a fascinating and easier one.*^^Let's read it to have a 'serious FUN'.
I realize the question is somewhat strange and the answer depends on what people work on. Let's say it is not ML/AI.
I've enjoyed e.g. 'Calculus' by G. Simmons and Niven's 'The Mathematics of Choice' even though I didn't apply any of that at work.
"Mathematics of Choice: How to Count Without Counting" by Ivan Morton Niven
https://www.amazon.com/Mathematics-Choice-New-Mathematical-L...