Ray is a good source of MM and Horrible Histories:

http://www.horriblebooks.com/

Some of the later books are really advanced IMO. Our DS6/second grader loves the humor and has probably read about half of them. When you get into the algebra books, they're definitely challenging (this coming from a person with a very strong math background).

We provide them and horrible histories as an "unschooling" outlet.

JB