The Lady at the Gate. Twilight Society Mysteries, #1

She's come here to die. When Lois Grover moves into the Beacon of Hope Assisted Living Center, she fears boredom will consume her, but a fellow resident soon invites her to a secret meeting of the Twilight Society. This quirky group of elderly residents, with a variety of skills, talents, and expertise, helps lost souls, tethered to the world of the living, by solving the mysteries keeping them from moving on.
The center itself-a historic colonial mansion that has functioned as a hospital, a school, and a sanitarium-possesses mysteries of its own that both help and hinder the society's investigations. As Lois joins them on their latest case-a decades-old unsolved murder-she realizes that her new purpose may be more meaningful than she ever imagined.