A tremendous work. If you like old-school play, this volume covers a whole other arena of gameplay when your sessions grow past the first set of "dungeon" complexes in walking distance of their starting town.
What I love as a DM is that it's very comprehensive, exceedingly detailed, and because I am a seat-of-pants game master, it's easy to strip down that complexity and just finally get how hex-crawling is supposed to work. How can you get the players to expore and uncover a world that doesn't exist yet, but will still make sense as you generate it in play.
Not only that, it has all the rules for interacting with what they find in a MACRO way. Establish your own settlements. Expand ones you find. Expand the borders of exsiting domains. Go to full on war.
This is all stuff I've seen before in a ton of different seperate and way less well organized works. This compiles all of that high-level gameplay in one volume, makes it super simple to use the way OSE is known for. I don't even play OSE specifically, but this is suitable to any D&D esque fantasy adventure game up to like, 2nd edition-ish, IMO. I use it with Black Hack no problem.
This thing operates like a missing Dungeon Master Guide for "well the players are bored with dragons in the dungeons, and want to take over the world. Can't say enough good stuff about this material. Super usable. Super useful. Great material to help your gameplay and game mastering uncover an actual living, breathing interconnected world.
Note: 100% get the "filling in the blanks" companion to this too. So much value. I realized that I got this book as part of a bundle, and my above review may also include praise for that book too, as they work together and I read them all at once.