Down and Dirty: This is a compilation product. If you are happy sorting through multiple books looking up rules that you use every session, this isn't the book for you. If convenience is worth 3 dollars - it certainly is for me - then you'll love this book.
The book can be broken into three large sections.
The first, Exploration Rules, is a simple and straight-forward compilation of materials presented in earlier products.
The second, Kingdom Building, has a few nice new rules - new buildings, new events - as well as everything that you know and love from earlier products. It is well put together, easy to read, and flows in a logical way. This section, however, has the one major drawback of the book - namely, the "Kingdom Events" are connected seamlessly to earlier sections. I would have preferred that the material be presented in multiple chapters, so that I could give the Kingdom Building crunchiness to my players without giving them all of the events as well. This could be solved by printing the materials but, as another commentator observed, the thick black borders making printing burdensome.
Finally, there are a few pages worth of player handouts. Again, these are things we've seen before, but it *is* convenient to have all of this material in one place.
"Monsters of the River Nations" is a good book that has a nice spread of new antagonists, skewing slightly toward Animals (6) Magical Beasts (3) Plants (3) and Swarms (3). I'd have preferred a few more Fey, but it does provide a little for everybody.
As other commentators have mentioned, one of the best features of this book are the Humanoid Encounters, especially the Cursed Brethren and their Bandit King. It fits well with Pathfinder Adventure Path #31, and could easily serve as a modular replacement for the Stag Lord and his band for DMs who prefer a darker theme or greater challenge.
Other than the Humanoid Encounters, my favorite bit from this book is the "Frost Mite Swarm." These little beasties make swarms fearsome again, and the disease they carry is both frightening and flavorful. I know I would avoid the deep parts of the woods near the solstice if I had a chance of meeting these.
The art is decent - nothing to write home about, but doesn't detract from the book. The layout is standard for a monster manual. I'm taking away one star because I would have preferred to see a tighter ecological theme in a book covering a small geographical area - having Frost Mites and Piranhas in close proximity limits the viability of using all of the monsters and maintaining a sensible world. Altogether a good buy - well worth five dollars.