Fantasy novel by Emma Bull, concerned with the battle for dominance between the Seelie and Unseelie Courts of the fey.
Eddi, the lead character is a musician and singer. She is approached by a Phooka (a dog who can transform into a man) and told she has been chosen as champion for the Seelie Court. A mortal champion is needed, so that the fey can die - without a mortal on the field of combat the fey are immortal and the two factions are at a stalemate
The novel is concerned with music as much as magic as it follows Eddi's band from it's formation, in parallel with the plot about the progress of the otherworldly war.
Bull is a musician as well as a writer and it shows -- the music scenes are vivid and completely believeable. The characters are sympathetic and well-drawn and the plot is involving.
A novel well worth reading if you can find it.