How Big Is 43 Quintillion by Lynn Huggins-Cooper is an excellent book for Middle-Schoolers about numbers and math.
Another great book in the Beyond Rubiks Cube series, this book is overflowing with answers to questions like.....
When is 100 not 100?
What is the biggest number to ever be named?
Do you know how many hairs are on your head?
What was the most profitable movie ever made?