Showing The Deadly Truth by Helen McCloy

When Dr Basil Willing rents a small shack for a vacation on Long Island he becomes embroiled with his landlady, Claudia Bethune. Claudia wants to learn the secrets of her relatives and friends, so she steals a truth serum and holds a dinner party for her nearest and dearest.

In the early morning hours, as Dr Willing returns to his cottage, he sees what he thinks is a fire and investigates. He finds Claudia near death at the table and hears footsteps fading up the stairs. Someone didn't want Claudia to learn the truth about them, and soon Dr Willing finds himself a suspect in murder.