Darkthunder's Way - David Sullivan, #3

There are circles within circles, lands of magic and wonder that touch upon our own. But now war is on the horizon in the mystical realm of the Sidhe-a brutal clash of Faery against Faery that threatens to cross secret boundaries into the unsuspecting world of mortal men. Once more young David Sullivan has been called upon to do the bidding of Lugh Samildinach, Lord of Tir-Nan-Og. Together with three friends-one white, one red, and one immortal-David embarks on an astonishing journey through a perilous kingdom of brittle brightness.
But grim betrayal lies in the path of their quest for peace in the Otherworlds. And an inescapable doom in the terrifying judgment of Uktena, the great and hideous serpent.