The Monster's Ring, by Bruce Coville, 1982

If only Russell Crannaker could really turn into Frankenstein's monster on Halloween, and frighten off Eddie who keeps beating him up! When a wrong turn to escape the bully leads him to Mr. Elives' Magic Shop, Russel buys the Monster's Ring for a dollar, and gets a lot more than he bargained for. Upper elementary fantasy, with just the right blend of thrills, naughtiness, and cafeteria humor. Strongly recommended, a great choice for children just starting to read novels.