The 5th Gender
G. L. Carriger2019

A species that has no word for murder, has a murderer aboard their spaceship.

ALIEN

Tristol lives in exile. But he's built a life for himself aboard a human space station. He's even started to understand the complex nuances of human courting rituals.

Detective Hastion is finally flirting back

MURDER

Except that Tristol's beloved space station is unexpectedly contacted by the galoi­ - a xenophobic species with five genders, purple skin, and serious attitude. They need the help of a human detective because there's a murderer aboard their spaceship. Murder is so rare, the galoi don't even have a word for it.

Tristol knows this because he is galoi.

ROMANCE

Which means that he and Detective Hastion are on the case... together.