Creating a character with mental and psychological illness, with a phobia of open spaces is a great idea. A. J. Finn describes the main character's inner thoughts and feelings very well. We can understand her sadness, worry, guilt and depression. We know from all the events narrated by the main character that some are illusions, and some are reality. The question is, which ones are illusions and which ones are real? Some illusions are easy to predict, but other occurrences are a surprise to readers. Exciting and curious. It’s not surprising that this story was made into a movie.