Witness at Large — Mignon G. Eberhart

Blood On The Beach
They had gathered on the private island to enjoy the summer and each other's company. But that was before "the accident," before the pretty young girl called Sister found Tom on the pier, crouched over Mildred's lifeless and battered body. Now as the handful of friends hide evidence to protect one another, the killer in their midst plans his next move. Is he Tom, the man Sister has secretly loved for so long? Or Boyd, who has been a brother to her? The list of suspects is short, and with the killer closing in the tiny island is getting smaller all the time.

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An Unseen Murderer Is Among Us...
Terror walks the fog-shrouded island. The pretty young girl called Sister knows that violent murder has been done and will be done again. Who is the unseen killer? Can it possibly be Tom, the man she has loved in secret for so long? And what about the strange behavior of Boyd, who has been like a brother?

As she wonders, the long shadow of the murderer moves forward to strike once more...