This is a strange, low-key book. It doesn't much resemble SF or fantasy, although it works hard to make the memory-architecture of the Incrementalists feel like an interesting "magic system". Perhaps it's more of a cozy murder mystery?
My usual gag about Gene Wolfe is that, having written the last word on the Unreliable Narrator, he went on to invent the Incompetent Narrator. Now Brust has found the next step, the Narrator Whose Head Has Been Deliberately Screwed With. All our narration comes through Phil (the oldest immortal) and Renee (new recruit), but if you're expecting a traditional rookie-eye info-dump, guess again. Renee winds up implanted with the personality of the dead immortal Celeste, which means that by chapter 5 she's explaining things to Phil. Only Celeste is more involved in the situation than either of them initially suspect. Quite a lot of the storyline involves recalling first-person narration and trying to figure out who has influenced it, and how. Do you see what I mean?
Let me put this a different way: the book is, among other things, a romance between Phil and Ren. It is, to my mind, a singularly unconvincing romance. But is there a reason for that? It turns out that there is. Maybe that doesn't reconcile you to reading the thing, but you can't say the authors didn't think about the question.
So: very sneaky use of narrative convention. Whether that makes the book... I enjoyed it but wasn't swept away. Unsurprisingly, I worry that I've just missed some fraction of the point. (Wolfe has the same effect on me.)
The metatextual twist at the end hardly requires comment; Brust can do that sort of stuff in his sleep. (I don't mean to neglect Skyler White, his co-author; I just don't know anything about her beyond this book.) Browse through incrementalists.org if the mood strikes you. I approve of the challenge ("what can you do to make the world suck less?") but I'm not sure how the book (and site) can help such a mentality snowball. But then, on the third hand, can I complain that the authors are trying?