Huh. I had no idea!
A Monster Calls was first a book, and now includes more than one, a glorious illustrated edition and 'making of' among them.
Here are the two things I need to know, and didn't before seeing the film:
1. According to some, director J.A. Bayona is the next Steven Spielberg, and this is his E.T. The latter is not quite the case, though it is a very good film, yes.
2. The watercolor illustrations by Felicity Jones' character, Lizzie (a.k.a. "Mum") are astonishing. They reminded me of Guillermo del Toro's work (love!), and the end credits confirm the link: del Toro is listed among the "Thanks." The illustrations themselves appear to be by the hand of one Juan Ramón Pou. Anyway, they are to die for....