The Long Secret by Louise Fitzhugh

Harriet the Spy refuses to become ruffled when an unidentified person starts leaving disturbing notes all over the quiet little beach town of Water Mill. She’s determined to discover the author of the notes. And she drags her friend, mousy Beth Ellen, into all kinds of odd and embarrassing situations in her efforts to reveal the culprit. Observing in her own special, caustic way with her ever-present notebook, Harriet the Spy is on the case. But will she be ready to face the truth when she finds it?