Mr. Zero — Patricia Wentworth

A high society house party in the country sets the stage for blackmail, sabotage, illicit passion, and murder in this witty, classic British mystery with a touch of political intrigue
 
The last thing London debutante Gay Hardwicke wants is to become involved in another one of her cousin's messy jams. But Lady Sylvia Colesborough is in over her head this time: She claims she's being blackmailed by the mysterious Mr. Zero. If she doesn't steal important government papers from the safe of her politically connected husband, the distinguished Sir Francis, her gambling debts will be exposed. Gay's reluctant attempts to help Sylvia backfire when a murder raises the stakes -- and Gay's boyfriend, Algy Somers, becomes the prime suspect.
 
Enlisting the aid of her society friends, Gay races to clear Algy's name and catch a cagey killer before her own days are numbered.