Looking for a book from the 70's about a bratty kid who was always getting in trouble. One day while climbing a tree he falls out and is saved by his "guardian devil". He jokes that he will call this devil "Bub" for short and "Beelzebubbaroo" for long".
This is The Big Joke Game by Scott Corbett. For the record, it's not "Beelzebubbaroo", it's "Beelzebubaroo"
A young boy who loves practical jokes and games finds himself in the strange land of Limbo where the only way out is to play a complicated game.