Treason at Lisson Grove : a Charlotte and Thomas Pitt novel / Anne Perry.

Enhanced descriptions from Syndetics:

Anne Perry's very special world of mystery, passion, and danger has attracted an entire generation of readers to her bestselling Victorian novels. Treason at Lisson Grove, her first Thomas and Charlotte Pitt novel in three years, is a masterpiece, inspired by history and spinning on a razor's edge of tension, with a cast of characters as rich as the universe Perry evokes.  

The man who lies bleeding to death in a London brickyard is no ordinary drifter but a secret informant prepared to divulge details of a potentially devastating international plot against the British government. Special Branch officer Thomas Pitt, hastening to rendezvous with him, arrives a second too late, preceded by a knife-wielding assassin. As the mortally wounded man's life slips away, so too does the information Pitt desperately needs. The killer in turn flees on an erratic course that leads Pitt in wild pursuit, from London's cobblestone streets to picturesque St. Malo on the French coast.

Meanwhile, Pitt's supervisor, the formidable Victor Narraway, finds himself accused of embezzling government funds. With Pitt incommunicado in France, Narraway turns to Pitt's clever wife, Charlotte, for help. The man who badmouthed Narraway and ruined his career with innuendo can be found in Ireland--so Charlotte agrees to pose as Narraway's sister and accompany him to Dublin to investigate.
 
But unknown to Pitt and Narraway, a shadowy plotter is setting a trap that, once sprung, could destroy not just reputations but the British empire itself.