This was the very first Diana Wynne Jones book I ever read, and since reading it she has been one of my very favourite authors.
Although this is not the first book in the Chrestomanci series, I would recommend it whether you have read the others or not, as the story works just as well as a stand alone as part of the series. (Although I certainly recommend the rest of the series too! They are all brilliant.)
Although this is a children's book, I think it can be enjoyed by all ages. Charming and believable characters, fantastic witchcraft - what more does a good book need! The style of writing, the manner or the witchcraft and the realistic characters made it all very believable and absorbing. Couldn't put it down!