The Poop Thief — Kristine Kathryn Rusch

Portia Meadows runs one of the few pet stores that sells familiars to the magical. Familiars -- delicate, moody creatures -- keep magic clean and pure. To lose a familiar means losing magic. And on a bright afternoon, Portia's assistant discovers that something essential has disappeared, threatening not just the magical within the store, but throughout the world.

“Rusch's short fiction is golden.”
-- Kansas City Star