Dead of night / Simon Scarrow.

Enhanced descriptions from Syndetics:

As Germany strangles under the tight grip of the Nazi Party, the frozen winter of 1940 brings even more reasons to fear the dark in the crackling new WWII crime novel from #1 Sunday Times bestselling author Simon Scarrow--perfect for fans of Philip Kerr's Berlin Trilogy, Robert Ludlum, Andrew Gross, and William Christie . . .

Germany, 1940. The world holds its breath and hopes for peace. At home, the Nazi Party's grip on power tightens. Reasons to fear the dark are growing . . .

On a night in Berlin, during the most bitter winter in memory, SS doctor Manfred Schmesler is found in his study, dead from an apparent self-inflicted gunshot wound. The hurried and official version of the Reich is suicide.

Schmesler's widow doesn't believe it. At the risk of running afoul of the Gestapo, neither does Criminal Inspector Horst Schenke. The warnings to drop the investigation only compel Schenke to dig deeper. Then Schenke learns of the suspicious death of a child in a remote clinic outside Potsdam.

At first, the cases seem unconnected, but soon chilling links emerge that point to a terrifying secret. Schenke isn't the only one in jeopardy. So is everyone within his circle, including Schmesler's widow, who has a secret of her own.

Under a pitiless regime, how deep into hell are they willing to go to find the truth? And what will it take to make it out alive?