My 13 year old and I have through parts of the first three chapters of "An Illustrated Theory of Numbers". I would reckon that if your student is motivated and

My 13 year old and I have through parts of the first three chapters of "An Illustrated Theory of Numbers". I would reckon that if your student is motivated and at upper high school level, they would have the sophistication to go at it alone. It is just a beautiful book also, with lots of exercises and the associate website http://illustratedtheoryofnumbers.com/ http://illustratedtheoryofnumbers.com/ also has Python notebooks if they are into programming.