25 books • 4 series
Book #2: MATHadazzles Mind Stretch Puzzles
Book #3: MATHadazzles Mindstretch Puzzles
Book #4: MATHadazzles Mind Stretch Puzzles Volume 4
Book #5: MATHadazzles Mind Stretch Puzzles
Book #7: MATHadazzles Mind Stretch Puzzles Volume 7