Y.S. Lee - The Traitor in the Tunnel (The Agency #3)

Queen Victoria has a little problem: there's a petty thief at work in Buckingham Palace. Charged with discretion, the Agency puts quick-witted Mary Quinn on the case, where she must pose as a domestic while fending off the attentions of a feckless Prince of Wales. But when the prince witnesses the murder of one of his friends in an opium den, the potential for scandal looms large. And Mary faces an even more unsettling possibility: the accused killer, a Chinese sailor imprisoned in the Tower of London, shares a name with her long-lost father. Meanwhile, engineer James Easton, Mary's onetime paramour, is at work shoring up the sewers beneath the palace, where an unexpected tunnel seems to be very much in use. Can Mary and James trust each other (and put their simmering feelings aside) long enough to solve the mystery and protect the Royal Family? Hoist on your waders for Mary's most personal case yet, where the stakes couldn't be higher - and she has everything to lose.