I've just finished reading this to my daughter Celyn (11, too disabled to read for herself). 
 It's a fun story of Cat and his witchy sister Gwendolin living in a world that's almost ours but where witches and wizards exist and are well-known. 
 Like an enormous wedge of literature it's one of those adventures that would be over really quite quickly and with very little fun if the man (or woman) at the top just sat down and told everyone what was going on. Dumbledore could probably have reduced the Harry Potter series from seven long books to three short ones if he just told Harry things he knew that Harry needed to know. But let's not dwell on that. 
 At first glance it's quite a light-hearted tale centred around the true motivation of the eponymous Chrestomanci, a very dapper master-wizard who takes the siblings into his castle and family. It's written with Diana Wynne Jones' customary skill, energy, and imagination, and very enjoyable. 
 As things develop elements of the story takes a surprisingly dark tone, but the best children's books always carry an edge, and this edge serves to add interest in the finale! 
 Those seeking a 'magic-system' will be disappointed. DWJ's magic is magic and she doesn't share the rulebook in any of the four novels we've read so far. But that's how I like it and I've no complaints. 
 I've ordered book 2 on the strength of this one, so that's my recommendation right there. 
 Join my 3-emails-a-year newsletter #prizes