The Monster's Ring (Magic Shop Series)

Twist it once, you're horned and haired;
Twist it twice and fangs are bared;
Twist it thrice? No one has dared!

 
Russell is sure that the ring he gets at Mr. Elives' shop is just a silly magic trick, but he follows the instructions and twists the ring twice anyway--and becomes a monster!
 
Includes an author's note.