I'll be honest here, Revelation and Ascension, the first two novels by Drew Karpyshyn within the Mass Effect timeline and context, are quite nice sci-fi reading, they feel parallel, complementary and add something, sort of, to the game's lore. The same feeling can't be associated to Retribution, it becomes slow, the story forced, the whole setting pushy and just another opportunity to use the franchise and the characters of the previous novels. It qualifies as merchandising rather than a complementary novel.
There is something utterly artificial about the way the novel starts, how the plot tries to be assembled, how the characters attempt to fit into some pre-established idea of a larger context. Things almost feel as if on fast forward, jumping around in an attempt to keep attention. The end result is a novel that isn't nearly as good as the previous ones.
The way the old characters meet again, how they interact, how they become part of a pseudo-story, how the author tries to interwine events from the games, the actions or inactions of Commander Sheppard and the people at the Citadel. The shadow presence of the Illusive Man, the doings of the Collectors and the inevitable force that the Reapers are. All add up to nothing but known ingredients mixed with no true recipe in mind.
Once finished the novel leaves a feeling of nothingness, nothing memorable, nothing worth recalling, nothing worth noting for a Mass Effect fan or a sci-fi enthusiast. I imagine that the author, or at least someone in the marketing team, saw Retribution as a sort of semi-introduction for the next game in the now official trilogy of Commander Sheppard's adventures, sadly enough it comes up short in all possible levels. There is only one thing that can be rescued, one that probably isn't even necessary to highlight, the Illusive Man, whatever his intentions, is a very, very bad man and cares little for whomever he works with as long as his goals are met.
As for the book, can't say I would recommend it to anyone but an extreme Mass Effect fan who also likes all kinds of merchandise because that is all that the novel ends up being. Sad but true.