I'm looking for a book I read around 5th grade. It was science fiction. The story of a boy, who wanders into an area which a moose-like creature has surrounded with a force field. The moose's people are friendly and have terrible memories. Somewhere on earth, the moose found an egg-shaped object that he really liked. Now, he can't remember what it is, so he is having a contest. Various aliens appear to try to solve the problem. There were illustrations, one of a Sherlock Holmes character, whose head was separate from his body. In the end, the boy solves the problem by suggesting that earth itself is the object the moose is seeking.
Any help would be appreciated.