37 books • 3 series
Book #1: Riddles and Brain Teasers for Intelligent Kids
Book #2: Wacky Riddles and Brain Teasers For Clever Kids
Book #3: Challenging Riddles For Smart Kids
Book #4: Riddling Riddles For Incredibly Intelligent Kids
Book #4: 999 Fun and Challenging Riddles For Smart Kids (3 in 1)