Declare by Tim Powers (Supernatural/Spy novel, 2001).
This one is among the oddest and most unexpected examples of blending genres that I’ve ever come across. Declare effectively ties often unsparingly brutal espionage fiction (during World War II and the Cold War era that followed) together with supernatural horror (specifically an especially dark variation on Middle Eastern Djinn mythology). And many of the characters involved are actual historical figures (most prominently the infamous British spy and traitor Kim Philby).
The main fictional character, Andrew Hale, is another British agent whose career of dangerous undercover work often involves, parallels and dangerously intersects with that of the eventually notorious turncoat. In fact, the two eventually learn just how intimately their lives and destinies are entangled–not the least in their shared fascination with and desire for Elena, another fictional agent of shifting loyalties who proves as potentially deadly as any male operative.
The decades sprawling action carries these three key figures from Nazi-occupied Paris to a shattered and divided post-war Berlin, to the torture chambers of Soviet Moscow and throughout the Middle East, with a particular and recurrent focus on a deadly supernatural force the officially atheist Russian Communists aspire to control atop Mount Ararat (the fabled location of Noah’s Ark in eastern Turkey).
If this all sounds pretty weird, it is–but in a good, suspenseful way. Powers (a well-known author of offbeat SF/F fiction) does a masterful job of blending what may at first seem incompatible themes. The fun-house mirror contradictions of high-level spy activity (where nobody dares trust anyone else completely, where alliances can shift in an instant and whatever-works amorality prevails) has its own internal, fatalistic logic. In Powers’ hands, the super-secret supernatural threat proves very well suited to such an environment. In the end, he even ties everything together with the death-throws of the Soviet Union itself.
Strange and wondrous stuff this, often downbeat but ultimately satisfying.