This is book number 1 in the Imaginary Veterinary series.
When Ben Silverstein is sent to the rundown town of Buttonville to spend the summer with his grandfather, he's certain it will be the most boring vacation ever. That is, until his grandfathers cat brings home what looks like ... a baby dragon? Enter Pearl Petal, a local girl with an eye for adventure, who helps Ben take the wounded dragon to the only veterinarian in town-the mysterious Dr. Woo. No one knows where Dr. Woo came from or why she's moved into the old button factory and renamed it Dr. Woo's Worm Hospital. But as Ben and Pearl discover once they are inside, Dr. Woo's isn't a worm hospital at all-it's actually a secret hospital for Imaginary Creatures.