Russell Troy: Monster boy - Bruce Coville

Goodreads 3.78

Mr Elives' Magic Shop seems to appear from nowhere - and disappears again just as fast! Never in the same place twice, it sells only the most exclusive wares...A talking toad, anyone? A dragon's egg? There's plenty to choose from...When Russell buys a magic ring from weird Mr Elives, he's told it will turn him into a monster. Just the thing for school! Russell's sure it's all a stupid trick - until he whispers the chant, twists the ring and begins to sprout horns...At first it's fun changing from boy to monster and back again, but Russell hasn't paid proper attention to the instructions - and now he's gone too far...