The Swimming Pool — Mary Roberts Rinehart

Judith Chandler had always been the spoiled beauty of the family. But in her glamorous, jet-setting life, something had gone wrong. And now, for the first time she was afraid. Desperately afraid.

With a deadline to meet on her detective story, Lois had no patience with her sister's strange behavior. Except the mystery Lois was writing was not nearly as deadly as the mystery about to unfold. For ravishing, willful Judith was about to disappear from her locked bedroom.

And by the, pale light of morning, a woman's body would be found in the crystal blue depths of THE SWIMMING POOL