When I say realistic, I really just mean interesting visually and conceptually. I need to get back into reading, but all of the books I've found with sapient and diplomatic aliens in them only have humanoid ones or "godlike" aliens with no real form. In a perfect world, I'm looking for something that has multiple distinct and thought out aliens like Becky Chambers' Wayfarer series, but with designs and backstories as fun (or funner) as Animorphs or Space Opera.
But really I just want a story on an alien world that isn't almost completely barren or taken over by humans. Or honestly any book where an interesting, alien species that isn't humanoid or just big evil bugs plays a fairly large role, and we get to see some of their culture and how they work.
I know this post is pretty old, but I just saw it and have a great book or you. Its titled "Brigador" by Brad Buckmaster. The story takes place on a sci-fi world that is controlled by an isoationist government. Because of this, the technology is growing old and unreliable, and the people are poor and tired of the the way they live. It follows a squadron of mech pilots who are tasked with investagating a checkpoint that has been attacked. While they are heading towards the site, one of their commrads, who is an extreamly skilled pilot, goes rogue when a revolution begins. Thats all I am going to say about the plot because that's just the beginning and it is really worth a read.
P.S. The "aliens" are hostile offworld pilots in mechs that are a lot more advanced than the MC's, and the fight scenes are awesome! I hope you check it out and enjoy it.
Rocheworld by Doctor Robert L. Forward. The Flouwen are amorphous shape-changers that breath ammonia and make parts of the blobby bodies into appendages like lenses to see. And the world of the title is a double planet with two small worldlets that share the same atmosphere and ocean and orbit each other.
Another example would by the tri-sexed gaseous aliens who inhabit an adjacent parallel universe in Isaac Asimov's novel The Gods Themselves.