Melvin Burgess: Lady: My Life as a Bitch [2003] paperback

A sexually active, rebellious teenager is transformed into a confused and frightened dog by a tramp with magical powers. Sandra has a lot of growing up to do quickly if she is to survive as a scrounging stray in a rough dogs' world. She experiences all the physical aspects of being a dog, including running with a pack and going on heat. She must decide whether she wants to recover her humanity, and if so how to do it - doubly difficult as her brain is reduced to the scattered consciousness of adog.