Schemes & Scandals by Kelley Armstrong

It’s Mallory Atkinson’s first Christmas in Scotland. Victorian Scotland, that is. Also, as the twenty-first-century detective learns, Christmas really isn’t a thing in Victorian Scotland. It’s all about Hogmanay. But her boss, Dr. Duncan Gray, treats her to an early gift of tickets to the event of the season: a Charles Dickens reading. There, they bump into Lady Inglis—the lovely widow who has sent Gray sexy letters trying to entice him back to her bed.
Lady Inglis introduces Mallory to Dickens—the meeting of a lifetime—but in return she wants their help. She’s being blackmailed. Someone stole letters she wrote to another lover and is threatening to publish them.
Mallory isn’t sure what to make of Lady Inglis, but no woman deserves that, so she insists on taking the case with or without Gray’s help. Growing tension between them soon tells Mallory that Gray is hiding a secret of his own. She has until Hogmanay to uncover the blackmailer…and, hopefully, to put things right with Gray so they can enjoy the holiday together.

Note this is not a full-length novel. It’s a novella set after Disturbing the Dead.