If you go to the end of the page you will see a number of other books on math for gifted children, including the novels on mathematical fiction by Robert Black.
Problemoids focuses on developing and using problem-solving strategies, not on memorization or operations. The hints and solution sheets are designed to introduce, teach, and reinforce 19 problem-solving strategies.
The program provides students with means of self-checking the most difficult part of their work. It has three components: student workbooks called CHALLENGE, which include 50 problems and several levels of strategy-based hints for each problem; a separate solution sheet for each problem with strategies for solving the problems illustrated in detail so the students can check their work; and a teacher manual called MENTOR, which explains the program and which contains the answers to the problems. The manuals and solution sheets are designed to minimize the instructor’s preparation time.