I found this fair-condition softcover at a local Savers thrift shop, and nabbed it for another release copy. [The book came about when Ness was asked to take the characters and premise that Siobhan Dowd left behind when she died, and turn them into a story, one that he tried to make into a tale that she'd have liked.]
"The monster showed up after midnight. As they do. Young Conor was awake when it came." So opens the story, with the illustration showing a huge, shadowy figure lurching through a graveyard. Conor, age 13, has been having nightmares, but he's definitely awake now, to hear a strange voice calling his name, and to see the huge churchyard yew tree - standing outside his window, its branches forming a great and terrible face... [The illustration of *that* made *me* nervous, as in turn-all-the-lights-on.]
We learn that Conor has deep concerns, from a bully at school (whose calm demeanor makes his attacks even more disturbing) to the unwelcome sympathy of teachers and students over his mother's illness - and then there are the repeated visits by the monster. It says it will tell him three stories ("Stories are the wildest things of all... Stories chase and bite and hunt."), and then Conor will tell the monster a fourth - and if it isn't the truth, the monster will eat him alive...
The monster's stories have unexpected elements which puzzle and anger Conor, and his reactions (or is it the monster's actions?) have devastating consequences. Seriously, the kid's going through hell, and the most well-meaning adults are just making things worse - some very deft psychological points made here. [Spoiler: The part where Conor's schoolmate Lily tells everyone about the seriousness of his mother's condition - triggering much of the behavior that gives him so much grief - struck me as a perfect example of a well-meant act that turned into a gross betrayal; when she realizes this and apologizes, it begins to turn things around for Conor, but for quite a while there, I hated poor Lily myself!]
The climax of the story, and the truth behind the monster's actions and Conor's secret, are quite harrowing and all too believable. I cried a good deal - and I really, really love this story.
[There's a short TV Tropes page for this book. I enjoyed the 2016 film adaptation, though I prefer the book.]