Peter Straub brilliantly blurs the line between reality and fiction in his latest thriller, “In the Night Room” (Random House, $21.95).

The novel is a sequel to “Lost Boy Lost Girl,” the story of Manhattan novelist Tim Underhill, who fictionalizes the disappearance of his teenage nephew, Mark, to ease the pain of losing him.

In his book, Tim allows Mark to slip away into an “Elsewhere,” accompanied by a beautiful phantom named Lucy Cleveland. In reality, Lucy is Lily Kalendar, the daughter of murderer Joseph Kalendar.

Tim awaits the publication of his novel (also titled “Lost Boy Lost Girl”) while working on a new manuscript. Trouble is, he’s seeing the ghost of his long-dead sister, April, who appears to be trying to warn him about something — and receiving eerie, fragmented e-mails from dead acquaintances.

Fears that his grip on sanity may be slipping aren’t lessened when young-adult novelist Willy Bryce Patrick shows up at one of Tim’s bookstore readings. Willy is running from her fiance, having discovered that he was probably responsible for the murder of her husband and young daughter.

Willy isn’t certain why she’s come to Tim for help, but her presence jolts him with pure fear. She’s one of his characters, who has somehow come to life.

As Tim slowly helps Willy realize that she’s not real, they travel to the Illinois town where Tim’s nephew disappeared. An abandoned house had been an obsession of Mark’s, and Tim had used it as the villain of his nephew’s disappearance.

In his novel, Tim had wrongly implied that Kalendar had killed his young daughter in that house. “His spirit, or whatever you want to call it, has been after me ever since he found out. He’s enraged” and wants restitution, Tim tells Willy.

“In the Night Room” is so imaginative, intricate and electrifying that readers will be tempted to race through the novel.

Don’t!