In the Vanishing Hour  by Sarah Beth Martin

Massachusetts, ​Summer, 1951: When a boy drowns in the Charles River, a family is devastated, and the town of Norumbega is changed forever.

Eight years later, Frances Adams lives in the shadow of her dead brother, Mac. When her new friend Gwen vanishes into the river, Frances becomes obsessed with her memory, and takes on her likeness. An investigation begins, and the police question three young men. Frances follows them to the local riverside park, where one of the men mistakes her for Gwen. Intrigued by the idea of haunting him, she continues her pursuit, immersing herself in the case. As the mystery unravels, shocking revelations about its connection to a long-past family tragedy come to the surface.

Set in the haunting atmosphere of 1950s and 1970s New England, In the Vanishing Hour weaves suspense and mystery into a story about loss, identity, and secrets best left untold.