I'm always hunting for books that treat aliens as more than just humans with weird foreheads or evil bugs to shoot. A lot of sci-fi uses them as a backdrop, but the ones that stick with me build entire societies with their own logic, taboos, and art. C.J. Cherryh's 'Foreigner' series is the gold standard here—it's a slow, meticulous deep dive into the atevi, where their biology dictates a social structure based on numerical associations, not emotional bonds. Trying to communicate across that gap is the whole story. Another good one is Becky Chambers' 'The Long Way to a Small, Angry Planet'; it's lighter in tone but the worldbuilding is so lived-in, from the reptilian Aandrisks and their complex clutches to the multi-gendered Grum. You get the sense these cultures existed long before the human character showed up.
Some readers bounce off that level of detail because the plot can feel secondary, but for me, that's the whole point. It's anthropology disguised as a novel. Even 'Children of Time' by Adrian Tchaikovsky, while focused on evolving spiders, does something similar—it constructs a non-human intelligence from the ground up, shaped by completely different pressures and biology. That's what I crave: aliens that feel genuinely alien, not just metaphors.