I'm going to go a completely different direction from other recommendations and say Concrete Mathematics by Knuth and Patashnik. They will definitely be able to use skills from analysis and calculus here, but there are so many additional tools in this book that it's very much a worthwhile digression. The marginal notes are great, as well!
When I saw the title, this was the first book that came to my mind. Very nicely written, possibly the best math book I've read (as a Computer Science MSc)
I own this book, and it's a favorite of mine.
https://www.amazon.com/Concrete-Mathematics-Foundation-Compu...