The premise of the novels is humanity has explored the solar system and colonized both Mars as well as the Sol Asteroid Belt. Unfortunately, this hasn't come with prosperity for all mankind. Poverty and conflict remain with humanity as the need for resources has become greater than ever. The conflict between the three factions has been made worse by the existence of the protomolecule, a billion-year-old alien artifact that has opened up new areas of technology as well as science. One of these is a massive celestial gate on the other end of the solar system.
A group of priests, priestesses, reverends, and other religious leaders have been assembled on a publicity mission to investigate it. Meanwhile, Clarissa Mao, daughter of Jules-Pierre Mao, plots to frame Captain Holden and the crew of the Rocinate in order to avenge her father. This frame-up job ends up forcing Holden and the crew through the gate to become the first people to see what lies across the universe.
The mystery is an interesting one as we get to see hints of what species created the protomolecule and why. The continued lack of actual aliens in the series is something which is both to its benefit and deterrent. It's really a series about humanity's reactions to alien life versus alien life itself. I don't know if we'll ever solve the mystery of what happened to them but it seems very likely we will and I'm not sure that's a great direction for the series to go.
An interesting element is the novel it is a surprisingly religious novel. The character of Anna is a devoted Eastern Orthodox priest (or so I believe--things can change a lot in Russia in 200 years) who wants to save the soul of Clarissa Mao as well as end the fighting without further bloodshed. She spends a lot of time contemplating God, the universe, destiny, and alien life which is not the sort of thing you usually find in hard science fiction novels.
The crew are decent in this book but nothing really interesting happens with them. Yes, Holden is framed but no one believes it for very long nor is there much tension from the crew. We also lack interactions with Bobbie Draper, which is a shame as I really liked her character. I will say that Holden is starting to grate on me as a character since his naked idealism only works with very cynical characters to contrast him to.
I also have to give the author's props for the fact they created the Behemoth--a converted Mormon generation ship which the OPA has turned into a completely useless military vessel. It can't fire any of its weapons due to the fact it's not structurally built for combat but it looks like it is. I will say, though, the book's handling on drug dealing offended me. One of the supposedly heroic characters spaces a man for dealing them and lost all sympathy as a result from me.
Indeed, my biggest issue with the book is the character of Bull. One of the major plots of the book is how he has to seize power from his insane military commander who is grossly underqualified for his position. When, in fact, I think Bull is a dangerously unstable murderer who mounts a mutiny for flimsy pretexts. When you actively hate one of the main characters and think he should fail, something has gone wrong.
In conclusion, Abaddon's Gate was....okay. I hope the next book is better, though. I just didn't feel the plot had the same level of tension, intrigue, politics, and twists which the previous books had. This was more like a tomb raid by the protagonists and I've never found the protomolecule or alien elements of the series as interesting as the political part. I compare it to A Song of Ice and Fire. I'm much more interested in the events in King's Landing versus the White Walkers.