The River Kingdoms is one of the more interesting settings for Golarion. Paizo does a nice job of keeping things detailed just enough to give creative GMs some leeway to add in some details of their own.
Being a big fan of China Mieville, I greatly appreciated his contributions in the description of the unique city state of Outsea.
There are some great sites of interest within one of the more standout areas of the campaign milieu that the PCs can explore. For example, a town that is a town but not a town... Can't say more as I don't wish to spoil it for any upcoming readers.
If you're thinking about using Paizo's next AP- Kingmaker, I strongly recommend purchasing this sourcebook.
I too wish there were more NPCs in the sourcebook but the material was an intriguing read.

This book deals with the River Kingdoms, an area crammed with independent states, countries, fiefdoms, cities, towns or even villages. Setting up such a patchwork allowed Paizo to bring a whole host of authors to write about 22 areas, each of them different and unique.

And the writing is, wow, excellent. From RPG veterans (John Wick, Chris Pramas, Richard Pett, the Paizo Crew), rising stars (RPG Superstar winners) to veterans of fiction (Elaine, China) the line-up is amazing and the quality rocks the house. The areas are unique, vivid and sometimes plain outlandish.

However, the book has a few shortcomings. Presentation of the chapters is not standardized - some authors provided city statblocks, flags/coats of arms while others kept to pure fluff. The map is somewhat unclear and lacks detail - a poster map would be a far better move.

Also, while the books is written with background material in mind, the amount of crunch is very, very small. There are next to no feats, traits, spells or equipment.

Overall - great writing and excellent ideas with some slight presentation issues and a dire need for more crunchy bits. Still, it's a fantastic book and I recommend it wholeheartedly.

I just received River Kingdoms and eagerly started to read it cover to cover. However, it seemed to me that yes, while it is a guide and a rich setting to be fleshed out in more details later, I would assume, the price tag to width and depth of the Kingdoms richness ratio is a little lackluster, sorry to say. Maybe to add more it needs more maps and biographies on some of the personalities in the Kingdoms? Just didn't seem up to par with the previous books for the price tag.