Philip Kerr's "Berlin Noir" trilogy has a deserved following. March Violets (1989) introduces Bernie Gunther, a Berlin cop in the 1930s. It's been more than a decade since I read the trilogy, so I'm foggy on the plots. Basically, Bernie hates the Nazis, ends up leaving the police, and becomes a private investigator. With people disappearing right and left after the Nazis come to power, Bernie has a lot of missing persons work. Bernie is a PI somewhat in the Marlowe tradition -- weary, moral, wisecracking, smart, and physical. He lands more women than Marlowe, though. The Nazis are essentially thugs and gangsters -- sort of organized criminals in power. One of the real appeals of Kerr's Gunther novels is the feel of time and place -- fear, rising antisemitism, and later, destroyed cities and desperate people. Historical figures play minor parts in the books.
Kerr put Bernie on a 15-year hiatus and wrote a bunch of other stuff, including a series of books for kids (maybe aiming to catch the Harry Potter wave). Several years ago, I read and enthusiastically reviewed (for The Oregonian) Dark Matter -- a crime novel with Isaac Newton as the central character.
Bernie reappeared a few years ago, but I only noticed when the second novel -- A Quiet Flame -- of the new cycle was released this year in the U.S. I read that one and backtracked to the first, The One From the Other. (The third, If the Dead Rise Not, is not available in the U.S. yet.) In A Quiet Flame, Bernie makes his way to Argentina along with other ex-SS men. (His reasons for fleeing are behind the plot in The One From...) He is hired by an official in Argentine security to find a missing girl after another girl has been gruesomely killed; the killing seems to copy two memorable murders in Germany in the 1930s. Bernie is off and running, delving into the world of ex-Nazis in Argentina, and even pursuing rumors of an Argentine death camp.
Both Flame and The One rely on complicated, multilevel subterfuge. Of course, traditionally, PIs are misdirected by their clients. Especially in The One, the plotting involves a con game of sorts that puts a little strain on the reading experience. Both books are compelling, but Flame is more satisfying. If you haven't read any of the Gunther novels, start with March Violets.