The Grass Widow's Tale — Ellis Peters

Ignored on her birthday by her husband and son, Bunty Felse takes herself out to celebrate -- and finds herself in mortal danger.

On the eve of her forty-first birthday, Bunty Felse is overcome with depression. The weather is dreary; her only child, Dominic, fails to call with birthday greetings; and her husband, George, arrives home only to announce that he has to leave for London immediately to attend to urgent police business. After almost twenty years as a detective's wife, Bunty doesn't protest or complain; she sends George off with a swiftly packed case.

To shake off her black mood, Bunty goes out for a solitary evening walk. She stops at the local pub for a drink and accepts a lift home from a sad young man whose troubles draw her out of her own and makes her feel compelled to help him. But as soon as the car door closes, the driver reveals a dark secret that could lead them both to early graves. Will she manage to escape the mysterious fugitive before it's too late?
 
The Grass Widow's Tale is the 7th book in the Felse Investigations, but you may enjoy reading the series in any order.