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. A lot of her books could be modeled on D&C 121 now that I think about it.
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. David stayed up way to late to read it. I have rubbed off on him. I read a bit less than I used to and he reads way more. And we both get stuck in books now instead of just me. So I try to get things that won't make me want to stay up until all hours of the night to finish. It is too hard the next morning.
The Pinhoe Egg. Dianna Wynne Jones. Eos Books. 2006