This is the newest book in the Chrestomanci series by Dianna Wynne Jones. I really like her books. They are technically YA books, so have to make an effort to find them but its usually worth it.
She has good plotting and realistic characters, for a book about magic at least. She tends to deal with issues pretty squarely too, a nice middle ground in a category (YA) that either tends to gloss over bad things or dwell on them in the interests of "realism."
The Chrestomanci is an enchanter who is in charge of magical law-keeping. As the ultimate authority in magical things he is a good foil for talking about ways of dealing with authority. Someone in most of these books needs help but is too afraid of authority for various reasons to go to the person who could help them. Aimed at teenagers it is a good theme I think. But it also talks about the things adults do to undermine the trust and authority they have.
In this one there are communities of witches who have stayed under the official radar for hundreds of years and a lot of the effort in the communities goes into avoiding the Chrestomanci's gaze. Since they live very close to him that takes some doing. Then the matriarch of one of the families goes a bit batty, dementia we assume, and things get really bad. It makes you glad we don't have magic in real life. The idea of a sorcerer getting Alzheimer's or a stroke is frightening.
A good story, I enjoyed it. ( )

The Gammer of the Pinhoe family of witches has finally lost her mind. Literally, lost her mind. Marianne believes that the Gaffer of the Farleigh's (another family) was responsible, but no one else believes her. Meanwhile, in Chrestomanci Castle, Cat's friend, Jason, is getting married. When Jason and his wife, Irene, go looking for a house, they bring Cat along. In the last house they look at, Cat sees Marianne and helps her find her cat, Nutcase. They find the cat, but Cat comes home with, what he suspects to be, an egg. A few days later the egg hatches and out pops a baby griffen, one of the rarest beast in the related worlds. Together with Marianne, he discovers some mightily interesting secrets about the Pinhoe family. All the while trying to keep Chrestomanci, the most powerful enchanter in the world, from finding out about what he's doing.

Pages-480. The Gammer of the Pinhoe family of witches has finally lost her mind. Literally, lost her mind. Marianne believes that the Gaffer of the Farleigh's (another family) was responsible, but no one else believes her. Meanwhile, in Chrestomanci Castle, Cat's friend, Jason, is getting married. When Jason and his wife, Irene, go looking for a house, they bring Cat along. In the last house they look at, Cat sees Marianne and helps her find her cat, Nutcase. They find the cat, but Cat comes home with, what he suspects to be, an egg. A few days later the egg hatches and out pops a baby griffen, one of the rarest beast in the related worlds. Together with Marianne, he discovers some mightily interesting secrets about the Pinhoe family. All the while trying to keep Chrestomanci, the most powerful enchanter in the world, from finding out about what he's doing. ( )

Marianne Pinhoe doesn't want to be Gammer. Joe Pinhoe doesn't want to be a boot boy at the Castle. Cat Chant is pretty sure he doesn't want a horse. Unfortunately, what you do and don't want doesn't make much difference when you're young enough to be bossed around by adults. When Gammer Pinhoe loses her mind and starts sending nasty spells to plague the neighboring Farleigh clan, things get out of hand pretty quickly.

A stasis bound egg, chronically escaping cat, infestations of frogs, and fleas, misdirecting paths, and imprisoned creatures round out the problems facing Joe, Marianne and Cat in Jones' latest Chrestomanci book. In typical Jones' fashion, everything isn't at all what it seems, but ultimately all the loose ends magically weave themselves together into one comprehensive pattern. ( )

Good. It's good to have another Cat story. I always liked him better than Christopher. ( )