Diana Wynne Jones: Eight Days of Luke [2000] paperback

There seemed nothing odd about Luke to begin with - except perhaps the snakes. If they were snakes, that is... David wasn't sure.

"Just kindle a flame and I'll be with you," says Luke. David thinks he's joking, but certainly, whenever he strikes a match, Luke appears immediately.

But David's new friend seems to have some extraordinary friends and relations, and some very dark secrets. And when David enters into a bargain with the mysterious one-eyed Mr Wedding, life gets very hot indeed!