Eight Days of Luke by Diana Wynne Jones

"Just kindle a flame and I'll be with you."

It's summer vacation, but David's miserably stuck with his unpleasant relatives. Then a strange boy named Luke turns up, charming and fun, joking that David has released him from a prison. Or is he joking? He certainly seems to have strange powers, and control over fire . . .

Luke has family problems of his own, and some very dark secrets. And when David agrees to a bargain with the mysterious Mr. Wedding, he finds himself in a dangerous hunt for a lost treasure, one that will determine Luke's fate!