The Fourth Stall by Chris Rylander

This is a difficult book to talk about because I feel it will not appeal to everyone.  Here is the publisher's description :

Diary of a Wimpy Kid meets The Sopranos in this 

laugh-a-minute mystery  from an exciting new talent."

"You need something?
I can get it for you.
You have a problem?
I can solve it.
That's why they come to me. By 'they' I mean every kid in the school.  First graders up to eight graders.  Everyone comes to me for help, and most of the time I am happy to provide it. For a small fee of course.
My office is located in the East Wing boys' bathroom, fourth stall from the high window.  My office hours are during early recess, lunch and afternoon recess."

Christian, known as Mac and his special team are running a very successful business, they are popular and have made plenty of money.  This money is needed so the boys can attend a special baseball game if their beloved team make it through to the finals.  Everything is going well until a little third grade boy comes to the office.  He calls himself 'Fred' and he tells Mac that Staples is after him.  Staples is a legendary high school-er famous for extortion and violence.  A gang war seems inevitable now that Staples has moved into Mac's territory.  

Here is a very detailed review of The Fourth Stall and the web site for Chris Rylander.  Finally here is a terrific little trailer.  You might also like to know there are two sequels which will arrive in our school library shortly.  I don't often limit a book by specifying an audience but I do think this is a book that will especially appeal to boys in grades 5 and 6.