The Mitford trial / Jessica Fellowes.

Enhanced descriptions from Syndetics:

A timeless murder mystery with the fascinating, glamorous Mitford sisters at its heart, The Mitford Trial is the fourth installment in the Mitford Murders series from Jessica Fellowes, inspired by a real-life murder in a story full of intrigue, affairs and betrayal...

It's lady's maid Louisa Cannon's wedding day, but the fantasy is shattered shortly after when she is approached by a secretive man asking her to spy on Diana Mitford--who is having an affair with the infamous Oswald Mosley--and her sister Unity.

Thus as summer 1933 dawns, Louisa finds herself accompanying the Mitfords on a glitzy cruise, full of the starriest members of Society. But the waters run red when a man is found attacked.

Back in London, the case is taken by lawyer Tom Mitford, and Louisa finds herself caught between worlds: of a love lost, a family divided, and a country caught in conflict.

Reviews provided by Syndetics

Library Journal Review

In Fellowes's The Mitford Trial, lady's maid Louisa Cannon is asked by a shadowy stranger to spy on Diana Mitford and her sister Unity, with someone ending up dead in the water when Louisa later accompanies the Mitfords on a cruise (75,000-copy first printing).

Publishers Weekly Review

In 1933, a few months after the Nazis take power in Germany, former Mitford family servant Louisa Cannon is approached by Iain, a British government agent, in bestseller Fellowes's subpar fourth Mitford Murders mystery (after 2020's The Mitford Scandal). Nancy Mitford, the eldest of the six Mitford sisters, recently offered to pay Louisa to accompany her mother and other family members on a Mediterranean cruise. Iain now asks her to accept Nancy's offer, because he wants her to spy on two Mitford sisters who have expressed support for the Nazis, notably Diana, who has been having an affair with English fascist Oswald Mosley. Louisa lies to her new husband, CID Det. Sgt. Guy Sullivan, telling him that she's going on the cruise to make some money. Guy ends up joining Louisa on the ship, where they're soon investigating a passenger's fatal bludgeoning. Flash-forwards to a murder trial with two defendants in 1935 inject some suspense, but this isn't enough to make up for an uninspired plot. Series fans will hope for a return to form next time. Agent: Caroline Michel, Peters Fraser and Dunlop (U.K.). (Jan.)

Booklist Review

This is the fourth Golden Age--style mystery (after The Mitford Scandal, 2020 to feature the six singular Mitford sisters and their ever-resourceful lady's maid, Louisa. The year is now 1933, and newlywed Louisa, studying stenography and settling into life as Mrs. Guy Sullivan, is recruited by MI-5 to keep an eye on the two fascist-leaning Mitford sisters who have fallen under the spell of Adolf Hitler. Louisa escorts Diana and Unity, along with their mother, Lady Redesdale, on an elegant Mediterranean cruise. Louisa's police-detective husband shows up to surprise her, but instead of enjoying a romantic tryst, they find themselves investigating a murder onboard while Louisa struggles to keep a sulking Unity, unhappy to be thwarted in a budding romance with a Nazi SS officer, in line. Inspired by the larger-than-life Mitford family and a real-life murder, Fellowes delivers a ripping-good read. Rich in detail, it moves steadily to its sad ending, leaving Louisa in possession of dark secrets that she will carry for the rest of her life.