Holes by Louis Sachar

Rating: Cleanest

Audience:  Fourth grade and up (though younger readers can enjoy).  I don't remember there being anything troublesome to watch out for.

Plot: Holes is about a boy named Stanley Yelnats  (whoa-ho, you see what he did there? S T A N L E Y Y E L N A T S), who has rotten luck thanks to a curse put on his ancestor.  After he is charged with a crime he did not commit, he is sentenced to time at "Camp Green Lake," a youth correctional facility in the middle of the desert.  The boys at Camp Green Lake dig holes in the desert every day. The counselors say it is to build character, but Stanley guesses that the warden is using the boys to search for something buried in the desert. Stanley soon discovers that he has to solve the mystery and right history to free himself and break his family's curse.

What makes it great?

Everything. When I was in elementary school, all the sixth graders were polled on their favorite book and Holes won with flying colors.  The plot is weird (in a good way), the characters are weird (in a better way), and the whole book is one big puzzle.  At first you don't see how everything fits together, but by the end, you're jumping up and down yelling "IT ALL MAKES SENSE NOW!"

A word on the movie:  The movie is great.  It's one of those few movies that actually follows the book, so I can highly recommend both.