I have read this novella three times now, with gaps of a few years between each go, most recently before I see the film (and this time I pictured the film's cast as the characters as I read). I still can't quite get a handle on the ending and hope the filmmakers' interpretation will help. It's a compulsive read and I'll probably read it again at some point. I definitely get more out of it each time.
The tension in this intriguing story is cleverly provided throughout the novel. The "unreliable" narrator" requires the reader to think about the events in the story - or even question them. A very beautifully written novel - that is the most important pleasure you get from reading it.