The Xenogenesis trilogy (Liliths Brood) by Octavia Butler. It examines what it means to be human and how much of us can be changed before we're no longer humans anymore. It also made me examine how we treat other cultures/species through the lens of how the aliens treat the humans and how they are so convinced that their way is the right way that they don't even question it until it's forcibly shoved in their faces. It shows the ugly side to the violent human agitators while also eposing the ugliness of the peaceful alien "saviors".

There's also a side thread of connections are chemicals in our brain and we can get trapped by them and circumstance into a situation where we're not always sure we're happy/on the right side/have any agency.

They aren't long books and even years later I still think about them sometimes.