One word kill
(Book)

Series title from Amazon.

In January 1986, fifteen-year-old boy-genius Nick Hayes discovers he's dying. And it isn't even the strangest thing to happen to him that week. Nick and his Dungeons & Dragons-playing friends are used to living in their imaginations. But when a new girl, Mia, joins the group and reality becomes weirder than the fantasy world they visit in their weekly games, none of them are prepared for what comes next. A strange--yet curiously familiar--man is following Nick, with abilities that just shouldn't exist. And this man bears a cryptic message: Mia's in grave danger, though she doesn't know it yet. She needs Nick's help--now. He finds himself in a race against time to unravel an impossible mystery and save the girl. And all that stands in his way is a probably terminal disease, a knife-wielding maniac and the laws of physics. Challenge accepted.