Ages 5-12. The only book I have found that addresses the concept of infinity, and does it well. Probably not an "again, again!" book, but great for that concept.

This book is not visually appealing as the illustrations are old-fashioned and lacking much colour, but the concept of infinity is addressed. What's the biggest number in the world? is a common question when children begin to grasp the concept of counting. This helps them understand that there is no one biggest number. I would read this book if I wanted to help a child understand that, but it isn't a "fun" bed-time story.