I thought this was a wonderful book and it was almost impossible to know what was going to happen next.
It's the the story of a pair of young orphans Cat and Gwen Chant who are sent to live with an old friend of their fathers after their parents death. They discover much about magic and themselves (at least Cat does) in this lively and well written book.
This is not the only book by this author set in this magical universe, but in some ways it's still my favourite after all these years. One more short story featuring Cat can be found in the author's book "mixed magics".
Eric (also known as "Cat") is someone with whom you sympathize from the very first page. His sister Gwendolyn is also sympathetic, but.... well, you'll find out.
There's a standard rule of drama that a new major character can't be introduced past the midpoint of a book. Whenever I hear that rule, I think of this book.... I don't think the author of that rule had this in mind!
One word of warning: This book is marketed as "the first book of the Chrestomanci series." But this isn't really a "series" at all -- these books really are independent of each other.