Fields of Blood is a book for the 'old' D&D 3.5 system, but it can work with Pathfinder very well. The book covers running a realm and fighting battles in d20, and it handles it very well. The rules for realms are clear and concise, offering a multitude of different styles of government and cultures that can be combined to produce a wide number of potential societies. You are given a number of options for things like resources (you need to stay solvent somehow, after all), guilds (thieves, wizards, etc) and the things they can do for you, temples that can bless your land in a variety of ways, and more.
The bigger part of the book is given over to mass combat. It gives you a system to raise and improve troops for yourself in a wide variety -- foot and horse and other sorts of cavalry, air troops, missile troops, specialists like combat engineers, monsters, etc. It also provides a system of mass combat that is among the better done in D&D, making combats exciting and quick and allowing for PCs to affect what happens.
The book ends with a few new prestige classes and spells, as well as guidelines on how a great many spells and magical items can be used to affect a realm's prosperity or the result of a battle.
It's an amazing value for the price, and if you want to run campaigns that involve ruling realms or warfare, you'll be happy that you got this one.
[5 of 5 Stars!]