This books is classic Diana Wynne Jones, and reminds me of why she was my favourite author as a child - and is still up there with the greats in my opinion now I'm an adult. Wynne Jones has that wonderful ability to make you care passionately about her characters within just a few pages. You're hooked from the very beginning, and this is a particularly page-turning story. I really enjoyed the descriptions of life as a servant in a big, traditional house, and the stuffiness of the establishment makes for many very funny moments. It's a lively, sparkling narrative, full of intrigue and action, and never dull. The protagonist, Conrad, is an instantly likable character, whilst the supercilious Christopher is just the right mixture of flaws and virtues. There's a host of fascinating and often amusing supporting characters as well.
Overall, it's a lighthearted story that is suitable for younger readers, but there are some real emotional undercurrents as well. Conrad's disinterested mother and his betrayal by those who are supposed to care for him echo similar themes in 'The Life of Christopher Chant' and 'Charmed Life'. There are also some subtle moments of irony that will be better appreciated by older readers. One is the scene where the butler excuses the bad behaviour of the heir and heirness of the manor due to their 'extreme youth' (twenties), all in the hearing of the much younger footmen (12 and 15) who are expected to work 18 hours a day and stand so still they are part of the furniture. Things do tie up neatly in the end, but I like that too about Wynne Jones' books - maybe it's not lifelike, but at least you don't get left frustrated by loose ends.
Whilst this book is part of the 'Chrestomanci series', it can be enjoyed without having read any of the others. The series is non-chronological anyway, so although this is the sixth to be published, the events are actually chronologically second. If you had a choice to read any of them first, then 'Lives of Christopher Chant' proceeds this one, and I suppose you could say there are some mild spoilers for that tale in this one.
I would highly recommend this, both as a children's book, and for adults who enjoy fantasy. Diana Wynne Jones' books are not the new Harry Potter. They preceeded Harry Potter by thirty years, and are in my opinion much much better. But if you enjoyed HP, you will almost certainly also like these.