One of Us Knows by Alyssa Cole

From the critically acclaimed and New York Times bestselling author of When No One Is Watching comes a riveting thriller about the new caretaker of a historic estate who finds herself trapped on an island with a murderer-and the ghosts of her past.
Years after a breakdown and a diagnosis of dissociative identity disorder derailed her historical preservationist career, Kenetria Nash and her alters have been given a second chance they can't refuse: a position as resident caretaker of a historic home. Having been dormant for years, Ken has no idea what led them to this isolated Hudson River island, but she's determined not to ruin their opportunity.
Then a surprise visit from the home's conservation trust just as a Nor'easter bears down on the island disrupts her newfound life, leaving Ken trapped with a group of possibly dangerous strangers-including the man who brought her life tumbling down years earlier. When he turns up dead, Ken is the prime suspect.
Caught in a web of secrets and in a race against time, Ken and her alters must band together to prove their innocence and discover the truth of Kavanaugh Island-and their own past-or they risk losing not only their future, but their life.