One of the things I like to do with series, though it definitely has an impact on how much I get to read overall, is re-read the previous volumes before I dive into the new one - as a result, I've spent the past couple of weeks reading all 3 of the Broken Earth books (The Fifth Season and The Obelisk Gate) and discovering all over again just how darn good they are!
When The Stone Sky starts, our main character, Essun, is living in a community that reluctantly accepts orogenes as members though this is still partly because she's used her power to make it clear she won't accept anything else. Meanwhile, her daughter Nassun has ironically developed a father-daughter relationship with Schaffa, who was the Guardian responsible for helping make Essun the powerful orogene she is. Essun wants to save the world and Nassun wants to destroy it, both working in what they see as the best interests of the people they love.
In this particular book, we also get our first storyline involving the origin of the stone eaters; it was mostly this that lost a star for me when I've given both previous volumes 5. I found those parts dragged a little and although otherwise this series hangs very well together as a whole, the sudden focus on them felt a little awkward. So, in that way, I found The Stone Sky the least engaging of the three books, though it's still substantially better written and more original than much of what's being published in the fantasy genre.
Will The Stone Sky do the treble and pick up the Best Novel Hugo, as its two previous volumes have? I really don't know, but it's still very much a possibility!