Blackout (Book 1 of 3) by Simon Scarrow

As the Nazis tighten their chokehold on the capital, panic and paranoia fester as blackout is rigidly enforced. Every night the city is plunged into an oppressive, suffocating darkness—pitch-perfect conditions for unspeakable acts.
 
When a young woman is found brutally murdered, it’s up to Criminal Inspector Horst Schenke to solve the case quickly. His reputation is already on the line for his failure to join the Nazi Party. If he doesn’t solve the case, the consequences could be fatal.  
 
Schenke's worst fears are confirmed when a second victim is found. As the investigation takes him deeper into the regime’s darkest corridors, Schenke realizes danger lurks behind every corner—and that the warring factions of the Reich can be as deadly as a killer stalking the streets . . .