Among the Mad by Jacqueline Winspear

Private detective Maisie Dobbs and her assistant Billy Beale are on their way home on Christmas Eve 1931 when they see a man commit suicide. The incident leaves her shaken and brings back memories of her service to England as a nurse during World War I. She is surprised to learn a few days later that her name is mentioned in a threat against the Prime Minister and the Home Secretary. After being cleared of any wrongdoing, Maisie is asked to assist the Special Branch of Scotland Yard before any violence occurs. Soon, it's obvious to Maisie that the letter writer is deranged and knowledgeable about deadly poisons. Maisie uses the connections that she's established over the years and diverges with the police in her opinion about the type of person responsible. Will Maisie be able to stand up for her beliefs and stop a killer? Among the Mad will be published in February.