The sixth season of The Expanse draws on the sixth novel, Babylon's Ashes, as source material and sees the crew of the Rocinante joining forces with various factions to try to bring down Marco Inaros, still basking in the glow of his successful terror attacks on Earth in the previous season.
The TV version of The Expanse is ending early, with the final three novels in the series going un-adapted. It appears that the plan is to turn Babylon's Ashes into a conclusion for this version of the series, with a view to adapting the final three novels (Persepolis Rising, Tiamat's Wrath, Leviathan Falls) in a different format later on. The last three novels are set thirty years after the rest with a significantly different plot focus, which might be easier to handle as a spin-off series or series of TV movies.
The final novel in the Expanse book series, Leviathan Falls, will be released on 30 November this year.