There are just a few authors that my 12 year old and I trust implicitly.
After having raced through umpteen of her novels, we may have placed Diana Wynne Jones in that category. Sure, The Magicians of Caprona was kind of stupid.... But if you locked us in a library, with a short deadline in which to emerge with a book we were willing to read, it might very well be one by Diana Wynne Jones.
Dogsbody pre-dates the Chrestomanci stories; it's a bit more science fiction than Jones' usual fantasy. The characters and plot -- Cinderella meets Puss (or, in this case, Dog) in Boots -- are very appealing.
The story is told mostly from the point of view of a high Illuminancy, Sirius, who, because he lost his temper and (apparently) killed someone, is exiled to Earth in the body of a new-born puppy. As Sirius learns how to survive as a dog, while solving the mystery of how he was framed, we also learn a bit about the Troubles in Northern Ireland and about how controling our impulses can help us get what we need/want.