The City of Lost Cats, by Tanya Lloyd Kyi

Left with a neglectful aunt who would rather rush off every evening to work at Municipal Hall, twelve year-old Fiona wants to do something more exciting this summer than the dance class she's been signed up for.  And when she stumbles across an abandoned old mansion full of feral cats, she thinks she's found it.  The mansion, referred to as 'The City" by its feline inhabitants has provided a community for strays and abandoned cats.  Fiona, abandoned by the adults around her is sympathetic to their plight and helps care for them.  

When she discovers that a dastardly developer (who also happens to be her aunt's boyfriend) has tricked Municipal Hall into giving him the permits to tear down the mansion and put up luxury apartments, Fiona and the cats leap into action to save the city of last cats. And while it would seem a child and a bunch of abandoned cats would not have the resources, they manage to save the day with the help of their local library and some sympathetic city workers.

An amusing light-hearted adventure of brave girls, adventuresome kitties, helpful librarians, a pair of workmen who love to demolish things, and two lost parakeets that speak in verse.  All of them come together in a ridiculously overlapping series of coincidences that lead to everything turning out perfectly in the end. While lacking much seriousness, the story does celebrate community activism in an easy going way that will appeal to cat-loving middle school readers.