Freaks And Shrieks — R.L. Stine

Wimp or Chimp!

Max made a deal with Nicky and Tara, the two ghosts who live in his bedroom: If he helps them figure out how they got turned into ghosts, they'll help him prove to his
dad that he isn't a worthless wimp.

Well, Max is about to make good on his end of the bargain. Someone saw what happened to the kids. Someone may know the secret that will bring them back to life! The problem is that the "someone" isn't a "someone" -- the "someone" is a chimpanzee! And Max is going to switch brains with him to learn the secret!