At that age, I enjoyed Number Theory by George E. Andrews

https://www.amazon.com/Number-Theory-Dover-Books-Mathematics...

There is an elementary number theory textbook by the famed number theorist and Ramanujan scholar Dr. George Andrews called Number Thoery, http://www.amazon.com/Number-Theory-Dover-Books-Mathematics/...

It contains proofs of very many results from elementary number theory, like Fermat's Little Theorem here. The reason this book is different from any other is because the proofs are all combinatorial, like counting beaded necklaces or otherwise. It is also a Dover book, meaning it costs very little. It is also one of the books I used as reference to create the elementary number theory course that I sometimes teach.