I like Diana Wynne Jones a lot as a rule, and this is one of her better-known books, but I don't think it's one of her best.
The plot revolves around the orphaned Cat and Gwendolen Chant, who taken to live with the mysterious Chrestomanci in a magical castle.
The characters are interesting enough but rather more two-dimensional than I'd come to expect from Jones and the plot annoyed me because it derives from one of my least favourite fictional tropes - People Not Telling Each Other Important Stuff. This is excusable as far as Cat and Gwendolen are concerned - they each have believable motives for keeping quiet. But the motivation for the adults involved keeping certain information secret seems flimsy at best.
Still worth reading but she's written much better stories.