Mysterious Benedict Society books are awesome. Super-genius kids find ways to outwit evil adults and help good ones. Adventurous, fun childhood fantasy that allows the reader to try to figure out the puzzles with the protagonist. THis three volume set on kindle is an unbelieveably good deal.
But, get the sample first. My son and I enjoyed the illustrations in the paperback books that we have. The Kindle version does not have the illustrations. Even, the Kindle version pricing is still a bargain. Hope to see more cool kids' collections on Kindle soon!
If your child likes this type of story, he/she might also enjoy the Tom Swift series - free on Gutenberg or the entire collection is formatted for the Kindle at a very low price through Amazon here: Tom Swift Collection (With easy click Table of Contents)