Lord John and the Private Matter by Diana Gabaldon

The year is 1757. On a clear morning in mid-June, Lord John Grey emerges from London’s Beefsteak Club, his mind in turmoil. A nobleman and a high-ranking officer in His Majesty’s army, Grey has just witnessed something shocking. But his efforts to avoid a scandal that might destroy his family are interrupted by something still more urgent: The Crown appoints him to investigate the brutal murder of a comrade-in-arms who may have been a traitor. Obliged to pursue two inquiries at once, Major Grey finds himself ensnared in a web of treachery and betrayal that touches every stratum of English society—and threatens all he holds dear.