Authority is the second book in the Southern Reach Trilogy (after Annihilation) and builds on this world. It remains a near-future, earth-based (probably) book, in which something has happened. The whole trilogy seems to be about discovering what that something is.
We have a new narrator, and they are also exploring this world “blind” so we learn the details of the world through their experience of learning and being introduced to it. In some ways this book feels less alien than the last, partly because we know the world and partly because the scenario is less contrived.
Basically this book is near-impossible to describe without lots of spoiler that would ruin it, but its well-written and I’m keen to get the last book in the trilogy.