An early masterpiece from Diana Wynne Jones. 'Dogsbody' has an unusually straight-forward plot for Jones. Sirius is a celestial. A being who is the embodiment of a star with various rights and powers which are never clearly explained (don't worry, you don't need them to be). The opening of the book is his sentencing for the murder of a fellow celestial. His punishment is to live a mortal life in the form of a dog on a planet where a weapon of great power has been lost. If Sirius can find that weapon in his mortal lifetime he'll be reinstated - if not, he dies forever.
Shortly after Sirius is born, with only fragmentary memories of who and what he is, his litter is sent to be drowned in the river. A few escape, and Sirius ends up being adopted into a family and loved in particular by a lonely girl named Kathleen. We're also treated to the conversations of cats and dogs and its delightful.
Jones had an unerring instinct for how children behave. As the book goes on the dynamic between Kathleen and her cousins and her tyrannical aunt is as compelling as the mystery that Sirius must solve if he is to survive. The story transforms from a fantasy/sf adventure into a heart-tugging story about kindness and friendship and the sacrifices we all end up making as we grow up.