The Little Stranger by Sarah Waters?

Yes, I understood it.

It's *supposed* to be ambiguous.

The point is that it's never really clear if there was a ghost at all, and if there is, how it manifested itself. But remember that the problems with the ghost only started when the doctor started coming to Hundreds, and that the doctor was rather obsessed with the house (and had been since childhood, and he had somewhat resented the family as well as being jealous). Could his subconscious have triggered the poltergeist activity? Could that be the significance of him seeing his own face in the mirror?

It's a bit like The Turn Of The Screw by Henry James - there are any number of different explanations for how things might have happened. It could be a real ghost. It could be imagined. It could be a real ghost manifesting itself around Caroline, or it could be manifesting itself around the doctor. Remember he was sitting right outside Hundreds in his car when Caroline died, and he was bitter that she had broken off the engagement. There are any number of possibilities, and the idea is that the reader has to make up their own mind.