The Secrets of Wishtide by Kate Saunders

In 1850, middle-aged widow Laetitia Rodd makes a living doing investigative work for her brother, Frederick, who is a prominent London lawyer. She is hired by Sir James Calderstone to investigate the past of the woman his son, James, wants to marry. Sir James disapproves of the match and wants the marriage stopped. Mrs. Rodd agrees to take on the case, since it will provide her with a good income, but soon she finds herself immersed in violence and is determined to prove the innocence of someone accused of murder.  The Secrets of Wishtide features a lively main character in this debut historical cozy mystery.