The Distant Hours by Kate Morton

The novel weaves a tale of mystery and family secrets, set against the backdrop of a decaying castle in the English countryside. When a long-lost letter arrives, it prompts Edie Burchill to delve into her mother's past and uncover the hidden history of the Blythe sisters, who reside in the enigmatic Milderhurst Castle. As Edie unravels the layers of the sisters' lives, she discovers a haunting story of love, betrayal, and madness that has been buried for decades. The narrative intricately explores themes of memory, the passage of time, and the enduring impact of the past on the present.