The Expanse is actually a pretty good book, telling the story of the episodes The Expanse amd The Xindi. It is very loyal to the script, but goes into more detail, and tells us how Archer was feeling about meeting a Xindi as well as how Trip felt about losing his sister. It also examines T'Pol's decision to leave the Vulcan High Command, and so provides a far greater depth to the characters.
My only complaint is that I would have liked a little more between scenes and extended scenes in the book, like the Broken Bow novel, but, overall, it even does this, including some deleted scenes from the episodes (a perticular highlight when I was reading it). In the end, The Expanse is a very enjoyable book that provides a greater insight into the characters at the start of their new mission.