The Shifting Tide — Anne Perry

William Monk knows London's streets like the back of his hand. But the river Thames and its teeming docks -- where wharf rats and night plunderers ply their trades -- is unknown territory. Only Monk's dire need for work persuades him to accept an assignment from shipping magnate Clement Louvain, to investigate the theft of a cargo of African ivory from Louvain's recently docked schooner, the Maude Idris. But why didn't Louvain report the ivory theft directly to the River Police? Even more mysterious is the desperately ill woman who Louvain claims is the discarded mistress of an old friend. As Monk endeavors to solve these riddles, he can't imagine the trap that will soon so fatefully ensnare him.