Remember Me by Mary Higgins Clark

Unable to forgive herself for the death of her two-year-old son Bobby in a car accident, Menley Nichols' marriage to Adam starts to fall apart - until the birth of their daughter Hannah. Determined to rebuild a life together around their precious baby, Menley and Adam decide to rent a house on Cape Cod for a month, confident that the tranquility of the place will be ideal for Menley and little Hannah. But the peace they crave is disturbed when strange things start to happen - incidents which make Menley relive the horror of the accident in which she lost Bobby. . . incidents which make her fear for Hannah. And step by step, Menley and Adam are drawn into a dark and sinister web of events which threatens their marriage, their child, and ultimately Menley's sanity.