The book is really made however by the illustrations of Jim Kay which are dark and atmospheric and really emphasise the fear and loneliness that Connor is feeling.
Here is the monster the first night it arrives in Connor's garden:
And when it arrives again in his grandmother's 'best' front room:
But my favourite is the image of the tiny tree sprouting from the knot in the floorboards after the monster's departure:A wonderful book, tackling a difficult subject very effectively; not just a story for children going through the loss of a parent or other family trauma, but a genuinely helpful book about expressing negative emotions. Probably suitable 8+ but I think a parent might want to read it themselves first to judge how their own child might respond to the ideas and images in the story.
(Linking back to my review of Michael Rosen's Sad Book, that also tackles the subject of death but aimed at much younger children.)