Eunuch of Stamboul by Dennis Wheatley (Vintage 1972 Illustrated Hardcover)

Moral young Englishman, Swithin Destime, to avoid an international incident, had resigned from the Army; within weeks he was in Istanbul to check on rumours of a planned uprising.

But as a spy Destime was an amateur, alone in a city of intrigue and fear, a dark web in whose centre squatted the repulsive form of Kazdim Hari Bekar.

Formerly a Palace eunuch, Kazdim was now Chief of the Secret Police – a job which admirably suited his depraved personal tastes