I really wish there were some kind of half-star system. It's not clear to me that Kerr's book is four whole stars, but it is quite good. For starters, it doesn't do a mystery any favors when I figure out the mystery pages and pages before the detective.
But then I suspect the actual detective story here isn't as important as the historical portrait the author is creating of 1937 Nazi Germany. Kerr's prose is evocative of time and place, though he does have, in this first novel, a blunt style of pointing at the historical specifics, giving us a street corner address, then informing us that it was such and such. This can be done better than it is done here.
There are some nice elements of Chandler-isms that don't read like the stale cliches that are SOP for detective satires. One favorite read, "I drove home feeling like a ventriloquist's mouth ucler." There's a lot of sad poetry in that single line. It's just a shame Kerr's slow detective didn't have a bit more in him.
(As one last note, the German Classic Romanticism that was one of Hitler's main, non-semitic obsessions, that sort of Young Werther thing, overlaps moodwise a bit uncomfortably with slouched hat, down-on-their-luck detectives. I'm hoping the next two books in his Berlin Noir
trilogy explore this a bit.)