Dangerous to Know by Tasha Alexander

Lady Emily is recuperating at her mother-in-law's home in Normandy after a traumatic honeymoon in Constantinople. While out riding one afternoon, she discovers the body of a young woman who has been brutally murdered. The woman, Edith Prier, had escaped from an asylum and is from a prominent local family. Emily finds herself drawn into the case, despite her husband's wishes that concentrate on recreational pursuits. Dangerous to Know is the fifth book in the Lady Emily series.