Now I Lay Me Down to Sleep by Patricia H. Rushford

After ten years of widowhood, Helen Bradley is happily remarried and working as a freelance travel writer, occasionally taking assignments for the U.S. government. Her life is disrupted when Irene Kincaid calls about the mysterious death of her husband, Dr. Paul Kincaid, a prominent gerontologist working on anti-aging products and an Alzheimer's cure.
When Irene is fatally shot, Helen suspects foul play tied to the Kincaid laboratories and checks herself into Dr. Kincaid's premier convalescent home, Edgewood Manor, ignoring her son's warnings.