Songs of Earth and Power Series by Greg Bear

The Song of Power opened the gateway to the Realm of the Sidhe, allowing young Michael Perrin to slip through. Now Michael faces years of captivity and deadly struggles for the future of the Realm and of Earth—leading finally to a terrible confrontation on the streets of Los Angeles, with the soul of humanity at stake.

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 - a gate that will lock behind you as you pass, barring you from the Earth forever. Resist at all cost. For it is a world of great danger and great beauty - and it is not good to be human in the Realm of the Sidhe.

When Michael is released from the land of the Sidhe, all he wants is to lead a normal life in L.A. But there are hauntings in the streets, bodies in a hotel, and an ancient creature summoned from a loch. Now Michael must become a Mage to save his country.