The 5th Gender by Gail Carriger

SPSFC

A species that has no word for murder has a murderer aboard their spaceship! New York Times bestselling romantic comedy author Gail Carriger (writing as G.L. Carriger) brings you a light-hearted science fiction mystery featuring an adorable lavender alien and his human crush. ALIEN Tristol lives in exile. But he’s built a life for himself aboard a massive space station. He’s even begun to understand the complex nuances of human courting rituals. Detective Hastion is finally flirting back! MURDER Except that Tristol’s beloved adopted home is unexpectedly contacted by the galoi – a xenophobic species with five genders, purple skin, and serious attitude. They need the help of a 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. "This is a fun, funny, sexy book, and I’m so happy it exists. It hits you in the feels, makes you laugh and cry, and makes you think about matters of family, kin, and the heart." ~ Bookriot