There are quite some changes, though I have to hand it to them that most of them are for the better.
The second novel compared to the first does somewhat feel like the force awakens feels when compared to a new hope, so mashing them together into basically a single crisis where the plots overlap was a very good move.
Also, zombie apocalypse on Eros would have been terribly cliche in our day and age. Really happy they decided to flush that bit.
What I'm less happy about are the changes in characterisation. Especially the rossi crew fell victim to "TV-dramatisation", where apparently the protagonists just can't have a healthy relationship with each other. It's a bit like Fox demanding of Joss Whedon that Wash and Zoe must have a troubled marriage, a nonsense which he succesfully defended against.
In the novels, the crew is the resting pole of the narrative. An island of peace and loyalty in a world going increasingly insane. It was those parts of the book that I enjoyed the most. But in the show, of course they all have to have their own agenda, keep secrets from each other and even point guns at each others heads, because... well, because, apparently.
Also not too happy with Bobby. I don't mind the casting, finding a 2m tall pitch-black actress that is also quite wide in the shoulders doesn't sound exactly easy.
But Bobby Draper in the books is a experienced professional soldier that is very good at what she does and therefore also knows that it's better for everyone if she doesn't have to do it, while the show made her a gung-ho rooky (in attitude, not in competence) with more testosterone than her male collegues, because that's what TV thinks female soldiers behave like. I don't mind that type of characterisation in general, so this is not at all about "whining about sjw's destroying xyz", it's just not Bobby. And I was looking forward to Bobby.
So yeah, I have mixed feelings about the faithfulness of the adaptation. It sure has the plot of the expanse, but in some areas, which happen to be rather important to me, it doesn't feel like the expanse.
This is why I try to never read the books before the films or TV shows come out. I find that people who read the book first are never happy with the adaptation.