Songs of Earth and Power by Greg Bear. This is really two books-- The Infinity Concerto and The Serpent Mage-- republished by Tor some years back in a single volume.

It's always painful to see a favorite author succumb to the Brain Eater, and worse still when you find that the effect was retroactive-- that older books you used to like have apparently been tampered with by some maniac with a Prose Portal, leaving you a volume of unreadable crap with a familiar cover, and a vague wish for Jurisfiction to restore the original. This happened with Robert Heinlein, whose books I used to enjoy, but now find nearly unreadable, and after reading Greg Bear's latest, I feared the same thing might be happening to him. Vitals read like an unfortunate hybrid of Michael Crichton and Tom Clancy, and Darwin's Radio wasn't a whole lot better.

As a result, I was hesitant to pick up this book, as the original two volumes meant a great deal to me, back in the day. It would've been painful to discover that this one had been eaten. Happily, when I was moved to pick it up after finding myself thinking about classical music (music figures prominently in the plot, as you might guess), I found it still intact.

The original two books are Bear's only fantasy novels, and as such, are radically different from the epic hard-SF disaster novels he's mostly known for. It's not quite as dramatic a difference as you see with John Barnes-- they're still disaster novels, in a sense, with the cause being magical, rather than scientific-- but it's a dramatic change in tone all the same.

Songs of Earth and Power follows the story of Michael Perrin, a teenager from Los Angeles who befriends Arno Waltiri, an aging composer of film scores, then finds himself caught up in magical events that determine the fate of whole worlds. Following directions given to Waltiri by the David Clarkham, a sinister and mysterious former collaborator, Michael stumbles across a boundary between worlds, and finds himself in the Realm of the Sidhe. Stuck in a world that's actively hostile to humankind, he must learn enough to keep himself alive, and ends up doing far more. By the end of the second book, he's not just trying to save himself, he's trying to save the whole world from disaster, as the Sidhe return to Earth.