Once again, Mitchell's Commissar Cain is front and center. It's no secret that the main conceit of the Cain series is that Cain is not the hero everyone thinks he is. He's no slouch in a battle, but he'd much rather be watching a fight from behind several layers of ceramite than participating in one. Cain is at least the second Black Library commissar that isn't what a commissar should be (the other one being Commissar Gaunt, whose books I usually enjoy). I appreciate that not every member of a particular group follows all of the characteristic rules of said group, but if piles of super-special-butterfly-commissars start popping up in Black Library fiction, they run the risk of diluting what commissars are supposed to represent within the greater mythos. Space Marines sometimes are depicted in a similarly troubling fashion.
The other element that's consistent across the series is the framing device that presents the story as part of a memoir being edited after Cain's death by one of his associates. Her voice comes mostly in little snippets of footnotes. Sometimes her understanding of events differs from Cain's, and though she acknowledges Cain's prowess on the battlefield--and even gives Cain more credit than he gives himself, at times--she's often exasperated by some of Cain's more unsavory personality flaws.
Between Cain's untrustworthy narration and his editor's interjections, the series has a different materiality than most Black Library novels. This is a book that's supposed to exist in the "real world" of 40K grimdark. It's an interesting change of pace, but sadly, the pace wears on a bit. We are constantly reminded that Cain looks out for himself and only reluctantly behaves heroically. We are constantly reminded of Cain's abuse of his "Hero of the Imperium" status. We are constantly reminded that Cain's adjutant smells bad (though he's a crack shot with a melta). The effect is a churning of phrases and ideas that feels like stalling at times.
The Greater Good packs in a lot of 40K odds and ends. We get Imperial Guard, Tau, Tyranids, Mechanicus, some space battling, and a wee smattering of Space Marines just to say they're there. Other than Cain and his assistant, though, none of the characters stand out too much.
The action is fine, though not stellar. There's a nice fight that features imaginative use of a tent. The climax feels a bit rushed, and I found it to be unsatisfying.
The Greater Good is an all right read, but don't expect it to stay with you much longer than the time it takes to put it back on the shelf.
I'll give it three sweaty gunners out of five.