Freak the Mighty, by Rodman Philbrick was first published in 1993. So, it may be a book that the target audience hasn't heard of before. Amazon lists it as being intended for ages nine and up. I would agree with this, though there are a few elements of the book that may be a bit intense for those on the younger end of the age range.
The story is about two boys, both somewhat outcasts in their own ways. Max is labeled as a slow learner and is rather tall and large for a middle school student. Kevin, on the other hand, has a medical condition that causes "his outside to stop growing while his insides keep growing." He's very small and walks with leg braces. He's also incredibly brilliant.
The two boys form an unlikely friendship, which eventually leads to Kevin (Freak) acting as the brain and Max acting as the body. Early on in the story, Max says that he never had a brain until Freak showed up. Max carries Kevin on his shoulders through many imaginary and real adventures. Together, they consider themselves to be Freak the Mighty.
Parents should know that the story contains a lot of heart. There are themes of acceptance and valuing others for who they are as people. But, there are also some intense moments when the boys encounter real criminals and are put in realistically dangerous situations. There are also some mentions of alcohol, smoking, and violence. Prior to the story beginning, Max's mother was murdered. This is not really a spoiler as it happened prior to where the story begins and is mentioned very early on in the book. Overall, those elements are not graphic and are as mild as they can be while still creating a sense of danger.
This is a book I plan to read with my 6th grade students and I'd mostly recommend it for middle school students. The story is told from Max's point of view and the language and style used is something that will catch the attention of readers from the first page.