SyFy Channel cancelled the series in early 2018 just before airing its third season. Later that year it was announced by show producers that Amazon had picked up the show for season four. It was also assisted by a fan campaign to save the show.
Chrisjen Avasarala doesn't appear until the second book in the series, but she has been added earlier to be able to portray the politics of the Expanse universe more deeply, as the politics were explained by internal monologue in the book.
In Season 3, Melba is wearing a jumpsuit branded from "Savage Industries". This has been confirmed by the writers as an in-show nod to Expanse superfan and behind-the-scenes host Adam Savage from Discovery Channel's Mythbusters.
The name of the ship, Rocinante, is the name of Don Quixote's horse from the novel Don Quixote by Miguel de Cervantes. The episode names Dulcinea and Windmills may also be allusions to the novel.
The Expanse is based on a book series of the same name written by James S A Corey, the pen name of Ty Franck and Daniel Abraham. It was originally developed as a tabletop RPG. But after having worked on the world building and fleshing out the major factions (Earth, Mars, the Belt) they felt it was too much information to leave to just a board game. And so they then developed it into a novel, which gave birth to a series.