I decided to do this review and include both of these novels as they finish up the Expanse Trilogy that I began with the earlier review of Leviathan Wakes a bit over a month back.
While S.A. Corey's initial novel in this series was very entertaining and a complete work in its own right, the follow ups are less baked. Leviathan Wakes saw the death of its strongest character, detective Miller, and the rest of the series suffers for it.
Caliban's War and Abaddon's Gate centers on Jim Holden and the other crewmembers of his ship. Holden is too "good" to be a main character...he is given no flaws other than his "goodness" and "honesty"...not real great flaws to generate action within a novel.
Additional problems arise in the story itself. While Leviathan Wakes largely focused on a traditional "missing person" hunt and alternated between two characters, the next two novels either repeat that same premise (the missing person) or wander off into an underdeveloped premise without anything remotely close to a satisfactory resolution while adding a bunch of weaker characters to the mix.
Both novels lose the impressive world building nature of Leviathan Wakes and no longer feel fresh while the invasion of the solar system by an alien weapon system dispatched billions of years ago, falls almost to the wayside in both works and the story focuses on almost solely the human to human intrigue--which is a large part of the unresolved nature of the entire series. Everything that Leviathan Wakes builds towards is left hanging in the next two novels with little learned and nothing gained.
I wish I could recommend these two novels and the series as a whole. The series isn't a total loss as there are certainly ideas and themes here that are valuable--particularly the "world" building contained within the initial novel. The descriptions of how humanity moved out into the solar system and the conflicts and machinations that occur with that move are extremely well done. If possible, the series might have been better hewing even closer to a more traditional military/whodunit style of works. The authors don't do a great job with their big driver of the series--the alien "protomolecule" and thus...it ultimately isn't worth it.