Mostly Ghostly: Freaks and Shrieks No.7  (English, Hardcover, Stine R. L.)

Max made a deal with Nicky and Tara, the two ghosts who live in his bedroom: If he helps them figure out how they turned into ghosts, they’ll help Max prove to his dad that he isn’t a worthless wimp.
Well, Max is about to make good on his promise. There’s a witness who saw what happened to the kids. A witness who may know the secret to bringing them back to life. The problem is the witness is a chimpanzee! And Max is going to switch brains with him to learn the secret. Will Max find the secret– or will he go from a worthless wimp to a worthless chimp?