Affinity by Sarah Waters
As the novel begins, the narrator, Margaret, makes a few references to an event that had happened two years ago around the time of her father's death. As a result, for most of the book, I doubted how reliable she was as the narrator since I felt like she was hiding something from the reader and merely hinting at "her illness."
It takes place in Victorian England, in the 1860s, and Margaret, coming from a rather well to do family, decides to give up some of her time to volunteer and visit inmates at the women's wing of a local prison. While there, she quickly becomes fascinated with one particular inmate, Selena Dawes, whose short journal entries are interspersed with Margaret's writings. Selena is in prison for fraud and assault, the event having been described in the first few pages of the novel. Before going to prison, Selena worked as a spiritualist, holding seances and so forth for some bored rich people. Margaret, who generally seems to be very rational, quickly begins to believe Selena's story and in the existence of spirits.
Since I had my doubts in the narrator, I kept looking for the logical explanation, and waiting for the mystery to finally explain itself, and the ending definitely didn't disappoint. Margaret is very smart and educated but given her restrictions as a female in her society, it is no surprise that she feels isolated and disconnected from her surroundings. Her loneliness thus makes her even more susceptible to any interest others might express in her.