I think in a lot of ways THE MOUNTAIN IN THE SEA is less about octopuses and more about late stage capitalism churning through the world and turning every living thing into ground up, extruded profits for stakeholders. Which does actually make for a good thriller--there's some great assassinations in here, and some desperate captives trying to manipulate an AI ship bent on catching every fish in the ocean. The writing voice is very 'thriller'. Contains lots of fascinating information and speculation about human sentience, alien sentience, octopus minds and even AI. I'm desperate to see more of the octopus folks!
I also am aware that I read this while in recovery from major surgery and so there's like an 80% chance I'll have to re-read it before I read a sequel, because I won't remember a thing. C'est la vie.