There are several books about Chrestomanci (including
Charmed Life,
The Lives of Christopher Chant, The Magicians of Caprona, Witch Week, Conrad's Fate, The Pinhoe Egg, and if there are any others I don't know what they are).
Now then, according to the books there are twelve different universes and this book is about the one next door (other than the death world). The Chrestomanci is their world's most powerful wizard and he controls the use of magic in that world. In this volume there are two books of the Chrestomanci books, the first one is about a boy named Cat Chant, and the second is about a boy called Christopher Chant.
Ok, so I was tired when I wrote this review, so I'll tell you more about this book.
The Chant boys are to be the next Chrestomanci after the old one dies, which is a long time since all of the Chrestomancis have nine lives. In Charmed Life, Cat (Eric) Chant doesn't seem to have any powers while his sister is passing tests on magic, so she dreams of ruling the world until a dark stranger appears and takes them to his house. In The Lives of Christopher Chant, Christopher can travel to other worlds and bring things back from the other worlds. When his Uncle Ralph discovers that, he uses Christopher's special powers to make a smuggling ring between the worlds, but when Christopher discovers that, he wonders what he could do to make every thing right again.