Ask HN: Recommend a maths book for a teenager?

It is a book produced by a remarkable cultural circumstance in the former Soviet Union which fostered the creation of groups of students, teachers, and mathematicians called "Mathematical Circles". The work is predicated on the idea that studying mathematics can generate the same enthusiasm as playing a team sport-without necessarily being competitive. This book is intended for both students and teachers who love mathematics and want to study its various branches beyond the limits of the school curriculum. It is also a book of mathematical recreations and, at the same time, a book containing vast theoretical and problem material in main areas of what authors consider to be "extracurricular mathematics".

https://artofproblemsolving.com/

There are many good recommendations here but I do think it will be good for them to gain some exposure to pure mathematics. It's different than what's typically taught in high school so they can start to get an idea whether they actually want to be a mathematician or instead focus on applied math in an engineering discipline.

Also you're probably going to get a computing bias here. I found the threads on physicsforums.com helpful so you might ask there as well if you want a different bias. (https://www.physicsforums.com/forums/science-and-math-textbo...)