If you have ever found yourself mapping a kingdom and wondering "How close to each others should this large cities be?" or "How wide would be the kingdom with this many people?", this is exactly the product you need.
In addition to that this book made me understand a lot more about how the medieval justice system works, how guilds fit into a (magical) medieval society, what is the hierarchy inside them...
Really, I can't recommend this book enough to anyone who wants to DM a medieval fantasy game. Be it 3rd or 4th edition DnD or some other system alltogether.