Girl at the Hanging Tree

Gemma Louise Coldiron has two personalities. One is herself-easygoing, curious, and travel-hungry. The other is a dictatorial other half named Tansy, who only lets Gemma come out of their Victorian mansion to do the shopping every two weeks.
Gemma also has amnesia, and Tansy's too busy painting macabre art to be forthcoming about their past. The town speaks to Gemma about a "Mr. Hardin," and it doesn't take her long to learn she was married to the oil tycoon before he mysteriously went missing.
While Gemma begs Tansy to tell her more about their life with the handsome stranger, Gemma soon learns that the town harbors a secret society. And Mr. Hardin? He was the leader.
If Gemma can't figure out what happened while maintaining her volatile relationship with Tansy, the town will assume Gemma killed Mr. Hardin-and even more alters will take root in Gemma's already crowded mind.
An acutely observed modern-day psychological thriller that plunges into the daunting abyss of mental illness, community indoctrination, and cult group-mind.