Who Guards the Prince? — Reginald Hill

Who would sever a tongue from a living mouth? Or kill a pathetic, homeless old man? Or terrify a young doctor into silence? The questions are piling up, and Doug McHarg can't stop asking them, even - especially - when he's warned off his inquiries by his boss in the local police force, by Scotland Yard, and by increasingly professional death-threats. The pattern that emerges is of a shadowy, immensely powerful organization, with a reach that extends to the White House and the English throne. And all that stands against them is the implacable McHarg - one discontented copper with little left to lose.