Most reviewers have concentrated on Roberts' commentary on wealth and gender, but I took something slightly different from it. By curious coincidence I have been reading this book on a trip to Tbilisi, which is the setting for a couple of scenes and the backdrop for several others. (I teased the author on Twitter about one geographical howler; the author replied that "it's possible the borders have been redrawn a little, in my future-world".) More to the point, Roberts' future world is also a world without conflict, where his characters (both rich and poor) are able to wander across borders that in our world are tense and contentious but in the world of By Light Alone are sunk into a sullen peace, watched over by local militias and strongmen whose desire for a quiet life apparently doesn't include conquering the next village. (Though the book ends with renewed conflict between rich and poor, personified in the family who are his core characters.)
Those of us who take an interest in the origins of conflict occasionally debate the extent to which access to resources is a universal factor (my own take is that it can be over-rated; cultural factors can exacerbate conflict even in areas which are wealthy, or prevent it in areas which are poor). Iain M. Banks portrays a post-scarcity future where conflict is pretty much absent except for those outside the Culture. I was a bit disappointed that the disappearance of traditional conflict from Roberts' world wasn't really a matter of comment within the novel; Tbilisi, Yerevan and Mount Ararat are basically far-off places which are not like New York and are full of poor people, and while that's explicitly the view of the unpleasant rich characters, I felt it was implicitly the view of the novel as a whole, and an opportunity missed.