This book is meant to be a resource, and it does that job excellently. There is a wealth of information in this book about all aspects of the life and functioning of various sized settlements in the pre-industrial world, and most of this information is going to be applicable whether you are creating something in a fantasy setting or working on a historical fiction setting.
The primary function here is to help make realistic and seemingly alive settlements for a game setting. As such it details the kinds of things that adventurers are going to interact with, and it does so in a rules agnostic way so that the information is usable in most any game system. Found here are various adventure hooks, for those times that your party inevitably wanders off from the thing you were planning on them doing and finds themselves befriending the random shop keeper you had to name gen during the drink break.
This is the kind of resource for a GM that is not happy with their towns being an inn and a ‘general store’ that totally has whatever cross section of the item list they decide they want the party to have access to. Here you will find loads of material aimed at fleshing out everything from that hamlet that the party stumbles into on the way to the mountain dungeons that are their true goal, to making the capital city of your setting feel grand, and gritty all at the same time.
Don’t think that this is a resource soley useful for a GM though, if you are doing any sort of worldbuilding in a fantasy or pre-industrial setting this will be something that will be useful to you. With the wealth of information in this book it will take a lot of the guesswork away about what types of goods, services, and personnel should be present and available in a settlement of any given size.