20 years after I first read this book, I *finally* tracked it down again. I remember being sufficiently scared at 11 that I couldn't read it at night. This time I finished it in an afternoon and the last 2 decades have taken nothing away from this superb book, indeed there are more subtleties now than on reading it as a child. It's clear that Marianne's dream world is a reflection - as dreams often are - of her fears and anxieties in real-life. She and Mark, the boy at the window, are both ill, and both almost fearful of leaving their sickbeds after months in them. In Mark's case, he is defeatist and "lazy" about getting well again, and believes there is no point in trying to walk, because he knows he won't be able to. All of this is conveyed to Marianne by a third party, her governess, since Mark and Marianne in real life have never met. Mark initially couldn't walk because of his illness - which corresponds to the lack of stairs in the dream house. When Marianne draws in the stairs (equal to Mark having the ability to walk in real-life, but not the inclination), Mark still refuses to use them. The evil boulders and menacing air, propelling them finally to try to leave the house, represent the medical opinion that if Mark and Marianne never try to get better, they will be sick for the rest of their lives: the boulders get closer and try to stop them leaving, but they must make the effort if they are finally to leave the house (dream) or sickbed (real life). Each dream sequence directly corresponds to what is going on in their real lives, and the escape to the safety of the tower and the sea symbolises their return to health - will they ever make it?
This book is eerie and menacing, and while I intend to watch the movie based on the book, PaperHouse, I'm a bit wary having read other reviews that seem to say the film has missed a large part of what the book is about and what it symbolises, and instead concentrates only on the horror aspect.