I read the Berlin Noir trilogy a few years ago, which contains the first 3 Bernie Gunther novels. I haven't read 4, 5, or 6, so I'm not sure what I missed. However, 'Field Gray' concentrates heavily on backstory.
The reader doesn't realize this at first, which I'm not sure was the best strategy. There's a great setup - Gunther ditching Havana (circa 1950s) on a cigarette boat with a sexy dame who just might be a wanted criminal... but all that is soon all-but-dropped, and we've flashed back to the 1930's.
The real focus of the book (we learn, as Gunther is interrogated, and thinks back to his past), is the relationship between Gunther and a man named Erich Mielke (an actual historical figure).
The facts (well, the fictional facts), on the face of it, seem unambiguously incriminating: Bernie Gunther was a member of the SS who repeatedly helped a man who was highly placed in the echelons of power, a murderer and a war criminal. But, as the reader learns, what actually happened was more nuanced, and much more complex.
Bernie Gunther is in some ways the quintessential noir investigator - hardboiled, tough and moody. It's still a bold move to have a Nazi (even a reluctant Nazi) as your hero; the Kerr pulls it off. I'd be happy to read the others in this series - I supposedly won the latest ('Man Without Breath') here on goodreads; but it hasn't arrived yet. Here's hoping it's on the way!