Lolly Willowes or the loving huntsman - Sylvia Townsend Warner

After the death of her father, Laura Willowes moves in with her brother and his wife where she is burdened with her new role as Aunt Lolly. After living with them for years and realizing her own unhappiness she suddenly decides to move to Great Mop, a small unassuming village in the countryside. There she makes a deal with the devil, a loving guardian and watcher-over of dissatisfied souls, and becomes a witch as she continues to fight off the persona of Aunt Lolly as it attempts to reclaim her. - Summary by Sandy