The Expanse is an interesting case of book-to-TV adaptation. The authors for the books were fairly involved with the TV series, and, in some ways, it's their retelling of the main story with some changes that streamlined things for the visual medium. The main things have to do with the consolidation of several characters (e.g., most prominently TV Drummer is an amalgam of three or four different people from the books), and the early introduction of some other ones (e.g., Avasarala and Draper) (though, on the flip side, because of the way actors contracts work, these characters were given make-work arcs in some seasons because they don't appear during the corresponding books). These changes generally made sense and were pretty well done.

Anyway, the books are excellent. The TV series is excellent.

Note that the last three books were not adapted for TV, though there was some set up that will eventually lead into those books. One logistical trick is that the last three take places some 30 years after the first six, so there's a matter of the actors' ages. But the TV series ended very well. You want more, but the main plot lines dominating the first six books were tied up.