R.L. Stine 
Night of the Werecat [EPUB ebook] 

She was warned… Wendy is excited when she spots the cat charm in Mrs. Bast’s booth. It will be the perfect addition to her cat collection. But Mrs. Bast refuses to sell it to her. She warns Wendy that it isn’t a cat charm — it is a werecat charm. And it is dangerous. Wendy doesn’t care. She has to have it. She grabs the charm, throws down some money, and runs. And that night — at the stroke of midnight — Wendy starts to change…