not-necessarily-textbook math books written by real mathematicians about real mathematics
with an unusual suggestion:
Skiena, Steven S. Calculated bets: Computers, gambling, and mathematical modeling to win. Cambridge University Press, 2001.
I read this book in one day, I found it so engaging. It uses quite a bit of mathematical modeling, but always in the pursuit of a practical goal (which he achieves): winning Jai-Alai bets against the odds.
(Calculated Bets, p.12)
A nice interaction between mathematics, statistics, gambling—and algorithms and practical programming.1 It could form a nice math-adult-supervised reading.
1 Skiena is the author of: *The Algorithm Design Manual*. Springer Science & Business Media, 1998.