Sly the Sleuth and the Pet Mysteries — Donna Jo Napoli

There are some strange new pets in Sly's neighborhood: a cat on a diet that keeps getting fatter, a fish that seems angry at its owner, and a peculiar pet named Wilson that suddenly disappears!
Only Sly the Sleuth can solve these mysteries. Grade-schooler by day, private detective by afternoon, Sly (a.k.a Sylvia) uses her sharp wits and reasoning skills to get to the bottom of these odd goings-on.
Donna Jo Napoli teams up with her son, Robert Furrow, and illustrator Heather Maione to create a heroine who is not only smart and spunky, but also tremendously funny.