A Monster Calls grabs you from the first page and doesn't let go until the end. How can you not keep reading after this:
He'd had a nightmare. Well, not a nightmare. The nightmare. The one he'd been having a lot lately. The one with the darkness and the wind and the screaming. (1.3)
It's the stuff of horror and mystery, and you want to find out what's causing the darkness and screaming just like you want to watch the end of The Exorcist. In this case, though, you probably won't need to peek out from under a blanket.
But Ness takes us out of Conor's concrete reality and into the world of magic right away when the monster shows up at his window and starts roaring at him. Didn't this kid just wake up from his nightmare? The reader is never sure just what plot twist is coming next; which world we're about to visit.
A Monster Calls is a short book, so Ness doesn't have time to go off on tangents. He keeps us in the action, steadily going back and forth between reality (Conor's issues at school and with his family) and alternate reality (the monster conjuring worlds in Conor's backyard.) The story is quick to read and easy to follow because of the fast-paced narrative.