A Dram of Poison, Charlotte Armstrong, White Lion Publishers, 1956, 221 pp
This mystery won the Edgar Award in 1957. It is unusual in that it is also a love story and a psychological portrait of a man finding his true nature.
Kenneth Gibson is a fifty-five year old bachelor leading a dull but comfortable and well-ordered life as a teacher. He is prone to helping people, especially fairly helpless characters. He takes on Rosemary, newly widowed and drowning in the fear of being alone. His attitude toward her is in a Henry Higgins mode but eventually they fall in love, despite a 23 year difference in age.
Bad things begin to happen, Kenneth becomes distraught and reckless, while the suspense builds inexorably until the final pages when all is resolved. Kenneth's psychopathic sister and deadly poison are involved.
Many intriguing and well-developed characters fill this very short book. In fact, the economy of the writing shows a master at work. Armstrong seamlessly incorporates philosophy and psychology into a unique take on a mystery.
(I could not find A Dram of Poison in any of my libraries. It is available from used book sellers.)