House of Zeor
House of Zeor is the first book published in the Sime~Gen series by Jacqueline Lichtenberg and Jean Lorrah, although later books occur earlier in the S~G timeline. It involves Hugh Valleroy, a "Gen" police officer (no connection to the relationship term; gen here is short for "generator") working across a political border, in tandem with a "Sime" leader, Klyd Farris, to track down a criminal and murderer in both nations. However, their cooperation is criminal in itself; if either government discovers them, they will both be executed.
The book shows Hugh's fascination with the quasi-vampiric simes and his growing trust of and friendship with Klyd as he begins to understand their biology and culture. Eventually, he must face a choice: Let Klyd die or risk his own life and allow himself to be used as food.
House of Zeor uses several buddy cop tropes - Hugh and Klyd have their differences, sometime extreme, but they are able to cooperate in the face of danger, and people often assume their relationship is closer than it is. Canonically, however, slash is not possible; Klyd assures Hugh that he is biologically incapable of being attracted to men.