Noose for a Lady — Gerald Verner

Sent to trial for the murder, by poison, of her husband John, Margaret Hallam is convicted and sentenced to death. Her appeal is dismissed and the Home Secretary refuses a reprieve. Simon Gale, an old friend of Margaret, returns from painting in Italy and learns about her case. Whilst refusing to believe she's guilty, the only way he can save her is to discover the real murderer's identity. But there are no concrete clues and time is against him...