If they like calculus and can stand proofs, I’d recommend a Course of Pure Mathematics by Hardy. It totally blew my mind when I was that age, to see how everyth

If they like calculus and can stand proofs, I’d recommend a Course of Pure Mathematics by Hardy. It totally blew my mind when I was that age, to see how everything was “connected” by proofs, starting with real numbers. Despite being proof heavy, I found the writing style singularly legible and comprehensible.