Boo by Neil Smith
"When Oliver "Boo" Dalrymple wakes up in heaven, the eighth-grade science geek thinks he died of a heart defect at his school. But soon after arriving in this hereafter reserved for dead thirteen-year-olds, Boo discovers he’s a 'gommer', a kid who was murdered. What’s more, his killer may also be in heaven. With help from the volatile Johnny, a classmate killed at the same school, Boo sets out to track down the mysterious Gunboy who cut short both their lives.
In a heartrending story written to his beloved parents, the odd but endearing Boo relates his astonishing heavenly adventures as he tests the limits of friendship, learns about forgiveness and, finally, makes peace with the boy he once was and the boy he can now be."
An okay book. The mystery is fairly easy to solve and there are parts in the middle that are just annoying. Not recommended.