I am interested in texts recomendations for a 14 years old boy who wants to study more mathematics than he does at school. He seems quite talented, but his knowledge of maths is rather low. I would prefer texts written by well known mathematicians. My first recommendation to him was Courant's book "What is mathematics?" for an introduction while I look for more systematic study texts. I am specially interested in an introductory study of calculus and elementary geometry (for introductions to number theory I think A. Baker or Hardy's books will fit). English, French or Spanish are ok.
Edit. Please note that I'm not interested in problem-solving books, nor lists of problems. Thank you anyway for those answers (Arnold's list seems interesting, even though for other purposes).