Infinity Concerto — Greg Bear

THERE IS A SONG YOU DARE NOT SING

... a melody that you dare not play, a concerto that you dare not hear It is called a Song of Power. It is a gateway to another world -- agate that will lock behind you as you pass, barring you from the Earth forever. When the Song calls to you, you must resist. For it is a world of great danger as well as great beauty -- and it as not good to be human in the Realm of the Sidhe.