Lord John and the Brotherhood of the Blade by Diana Gabaldon

Diana Gabaldon, the #1 New York Times bestselling author of the Outlander saga, brings back one of her most compelling Lord John Grey. Here Gabaldon weaves together the strands of Lord John’s secret and public lives—a shattering family mystery, a love affair with potentially disastrous consequences, and a war that stretches from the Old World to the New.
 
It’s been seventeen years since Lord John’s father, the Duke of Pardloe, was found dead, a pistol in his hand and accusations of his role as a Jacobite agent staining forever a family’s honor. Now unlaid ghosts from the past are stirring. Lord John’s brother has mysteriously received a page of their late father’s missing diary—and John is convinced that someone is taunting the Grey family with secrets from the grave. So he turns to the only man he can the Scottish Jacobite James Fraser. But war, a forbidden affair, and Fraser’s own secrets will complicate Lord John’s quest—until James Fraser yields the missing piece of an astounding puzzle and Lord John must decide whether his family’s honor is worth his life.