Mind Boggling Numbers by Michael J. Rosen
This is not the first book of this sort I've ever seen. Young readers are often fascinated by big numbers and it can be fun for authors to try to explain them. The comparrisons in this book can be a bit odd at times (how many whales to span a distance, how many pints of lemonade to fill a swimming pool) but where it really shines is in the math. Rosen fully explains the math used to find each answer. At the very least, this is a fun argument to give to the kid who claims that word problems are boring.
Mind Boggling Numbers releases September 1.