The word and the void - Terry Brooks

RUNNING WITH THE DEMON: One Fourth of July weekend, two men come to Hopewell, Illinois. One is a demon, a dark servant of the Void, who will feed off the anger and frustration of the community to achieve a terrible goal. Whilst families picnic and fireworks explode only John Ross, a Knight of the Word, can change the fate of humanity and save the soul of fourteen-year-old Nest Freemark.
A KNIGHT OF THE WORD: The years spent saving mankind from the horrors of the Void have taken their toll on John Ross. He's lost his spirit, his belief, and now, it seems his magic. When Nest is once again drawn into the mythic world of the Word and the Void she must warn John to take up his staff of magic once again, or the battle may be lost forever.
ANGEL FIRE EAST: For twenty five years, Ross has single-handedly battled against the legion of demons of the Void. But now he learns of a gypsy morph, whose power and wild magics will be an invaluable weapon against the Void - but the Void knows this too and will not rest until the creature has been corrupted - or destroyed.