I like the Bernie Gunther novels too, although they are set between the 1930s - the Nazi era, wartime and after, and Bernie gets involved in early Nazi hunting days. He's a cynical PI, and Philip Kerr obviously had a lot of fun pastiching Marlow. If you don't find Marlow pastiche funny it could be grating. There are 14 books in the series and if you use Kindle you can probably get bargains, more easily. Books 1-3 in the series are most easily available as a trilogy, in paperback or Kindle.
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Sadly Philip Kerr died in his early 60s a few years ago, but Jane Thynne obviously shared her husband's interest, and has written a number of books of her own, including 3 series of books
The first is the Clara Thynne books, featuring a young woman who is half German, half English, was brought up in England and has now come to Berlin.
The second features a woman in London being recruited as some kind of agent, and returning to places she knew before like Vienna and Paris.
Then there are two books set in alternate reality British Reich, written under another name - C J Carey I think - #1 is Widowland
And a recent standalone set in the 30s. She has also written a handful of other books before she joined Kerr in writing about this stuff. I wonder if they fought over reference books!