I don't think these two books exactly explain the largest number question. But they are good for young kids who are fascinated by math concepts.

How Much is a Million
http://www.amazon.com/Million-Anniversary-Edition-Reading-Rainbow/dp/0688099335

and by the same author
G is for Google
http://www.amazon.com/G-Googol-Math-Alphabet-Book/dp/1883672589/ref=asap_bc?ie=UTF8