A Lesson in Secrets by Jacqueline Winspear

It's 1932 and private detective Maisie Dobbs is asked to go undercover at the College of St. Francis in Cambridge by the British Secret Service. With a background in philosophy, they feel she's the perfect person to pose as a teacher while investigating any possible un-British activity at the pacifist college. When Greville Liddicote, the college's founder, is murdered, Maisie finds herself in the thick of two cases that might be related. While juggling her duties in Cambridge, Maisie is also trying to maintain a long distance relationship with James Compton, who's been in Canada on business. In addition, she's aiding an old acquaintance, Sandra, in getting back on her feet after her husband's death. She's also persuading her assistant, Billy, to move with his family to a brand new home that Maisie herself has bought (it involves some subterfuge to get Billy to accept her help without really knowing it). A Lesson in Secrets is one of the strongest books in the Maisie Dobbs series because of the ongoing character developments for Maisie and her circle of friends. It's also an interesting mystery that explores the rise of Nazism in Britain.