A Celtic harpist woman from the 600s (yes, 1400 years ago) finally broke free from the Faerie lands in the late 1700s. She spent the past 200 years trying to perfect her music and magic in order to go back to the Faerie gate and win freedom for her lover. As the book unfolds (in the 1980s) Christa discovers rock and roll, and bends her energy to building a rock band that can blast its way through to Faerie before the mortal realm and faerie realm diverge too far for her to bridge.
The music theory is alternatively a plot-carrying device and a weight on the plot. What is more important are the arcs of self-realization across which the various characters move (or descend) due to Christa's influence in their lives. Christa herself changes from the mild-persona harper back to the warrior she once was, and regains her fighting spirit while helping her friends fight for their own. Among them, Kevin was kicked out of his Irish Catholic family for playing the blues and lost his touch to the magic within the music; Monica was abused by her boyfriend; Melinda was -and becomes again- a drug addict... Christa has to face the fact that she cannot fight alone, and that there is a very real possibility that she will lose this fight.
The fundamental belief that women are warriors, valued participants in society, individuals, owners of their own destiny, etc. is a constant theme in this book. Baudino uses Christa to demonstrate that small can be spirited without being brass or pushy. She brings in religion and religious rites of passage to step her characters through their enlightenments without causing too much of a break in the story line. And she tucks smaller characters into the corners to prop up her feminist agenda with care as well as craven and cunning. (Thankfully, not every male persona aside from the hero is a jerk. That would have gotten really old really fast.)
Vengeance is a topic that is usually cut-and-dried in books. Someone does X, someone else goes out for revenge. Baudino has the same character arguing both sides of vengeance within a few short pages, though she skips the moral maudlin wallow that often occurs afterward. The third time, restitution and recognition are oddly understandable, and become a part of another character's discovery of something to do with music and life beyond self enjoyment.
Gael Baudino published this book in 1990, and it is very much an 80's book. It's one of the few fantasy books that I've read that includes a character who died of AIDS. The main characters deal with religion, homosexuality, close-minded families/bands, abuse histories, abusive relationships, stalkers, drug addiction, and self-esteem problems. It is now OOP, sadly, so if you want to read it, you might have to go hunting.
ETA: WOW. The GoodReads summary of the book is incredibly negative and - to my mind - hateful. I'm rather surprised. But please go read that for a different opinion than mine. It is here, or just behind the cut.
Despite a magical, nurturing Celtic lass at center stage, Baudino's (Dragonsword) rock 'n' roll fantasy flaunts its misanthropy. In sixth-century Ireland, Chairiste Ni Cummen and her female lover, Siudb Ni Corb, visit the home of the immortals to hear the bard Orfide play the harp. Chairiste escapes from the Realm, but Siudb is caught there. Fourteen centuries later, in 1987, Chairiste, now a harpist, is still trying to figure out how to save Siudb, when she discovers a different kind of music--heavy metal. Realizing the power of rock to blast open the portals of the Realm, Chairiste assembles an all-woman band called Gossamer Axe: Devi (sexually abused by her father), Lisa (exploited by men in her previous band), Monica (threatened and beaten by the male lover she left) and Melinda, whose drug habit nearly destroys the band (her male lover's fault, of course, for making the drugs available). With few exceptions, heterosexual males in this tale are hateful and destructive. The appearance of a gay man who dies of AIDS is a despicable exploitation of that disease. In Baudino's scenario, only women make beautiful music."