The Shadow Dancers: Jack L. Chalker

G.O.D. Inc II. Brandy and Sam Horowitz are private eyes who handle cases for G.O.D. Inc, who run the Labyrinth, the inter-world railway that connects the infinite, alternative Earths. G.O.D. is worried - there's a new drug in town, coming from off-Earth. It's instantly addictive and it takes over the brain - and what it wants, the user does. Brandy and Sam have to stop the drug reaching the client's home world. But the opposition got fake Sams and Brandys already set up on the two worlds that might hold any leads: one a world that's never heard of civil rights, and one ruled by Nazis. Which is really bad news for a black, Jewish couple!