The biggest difference I noticed between the first book and the show (aside from Avasarala and Earth in general not being in it at all) is that the ordering of events are a little different. Where in the show the Roci first finds the stealth ship, which then leads them to follow Julie to Eros; the books it's the other way around in that they go straight to Eros and that whole situation happens, and after they escape they backtrack Julie's course to the parked stealth ship and get the backstory and context for what they just witnessed.
Honestly, I think both versions of that is the right one for their respective medium, which to me is the telltale sign of an excellent adaptation. For a live action show, you want that clear and progressive ramping up of the stakes, so you first show the audience the stakes and save that whole mess on Eros for the finale where they get that sense of inevitable dread about what's about to go down. For a book, it's best to pace that stuff out and not frontload with too much exposition, having the revelations and backstory told after the fact helps pull the narrative along a little better. Plus you really don't want to rely on action too much in a book, as that is almost always the fastest way to bore one's readership.
Another vital difference is in the world building, especially regarding the belters. The book only gives you a little bit here and there, and maybe the odd Belter Creole phrase, but the show out of necessity had to develop it a lot more because of the realities of a live action production. A book can just describe the underbelly of Ceres in less than a paragraph, but a show has to make it. That means production design, graphic design for the signage, extras and bit players with dialogue need things to say and accents to say them in.
Oh and as for Jarred Harris coming back; I've been listening to
Ty Franck & Wes Chatham's podcast and I honestly wouldn't hold my breath. Apparently they've tried to get him back a few times over the past couple years but despite being more than willing to do so, he's been just so unbelievably busy and in such high demand of late that they can never seem to book him in time. Hopefully they managed to work it out for this last season, but I wouldn't be surprised if they just can't make it happen.
Honestly, the trope of "They were all ready to kill each other and then they became a family" thing they put into the show? Errrgh. Oh, and I can't stand Show Holden for a good chunk of the first season. Show Naomi is annoying too. Book Naomi is amazing. The show gets better the closer the characters get to the books. (This is why the Amos and Peaches story in season 5 was perfect.)
