The Sellen River cuts across eastern Avistan, all the way from the Lake of Mists and Vales in the north to Star Bay in the Inner Sea 1200 miles to the south. It passes through (or forms the borders of) numerous lands along its way. Amongst them are Numeria and the River Kingdoms, which are the main topic of People of the River, the latest release in the Pathfinder Player Companion line. It provides new options for players making characters from these lands, and also provides some rules and information regarding rivers in general.
The River Kingdoms are actually a grouping of numerous small kingdoms. Combined with Numeria, they make for a large amount of material for this one small book to cover. Not surprisingly, it can't cover them all and there are several River Kingdoms that get no more than a sentence or two of mention. As a Player Companion book, it also devotes a large amount of space to game options, like new traits and archetypes, further limiting just how much it can cover about these locations. In my review of the recent Numeria, Land of Fallen Stars, I stated that that book does a great job at describing what it is like to adventure in Numeria, but gives little information about what it's like to live there. Somewhat unfortunately, this book doesn't really fill in that gap. Players without much pre-existing knowledge of the lands covered in People of the River will come away from the book with only a smattering more knowledge than they started with. However, they will come away with several new options to consider for their characters, and for many players, that may well be more than enough.