Birds of a Feather by Jacqueline Winspear

This is the second in the Maisie Dobbs series.  Three women are killed and another is in hiding.  Their deaths seem unrelated until Maisie discovers white feathers in the places where each woman was killed.  There is always good history in these books and this window into post WWl England is no exception.  By the way, there really was an Order of the White Feather.