It's a series of quick sketches for you to fill in. It does drip atmosphere and your version of the area will be like no-one else's, but that's because there simply isn't enough detail for there to be much in common.

Eleven of the 126 pages are blank, and the main areas get four pages each: 'here's some ideas for the aesthetics of the area, here are three themes for games here, here are five one sentence ideas for adventures, here are five or six character outlines (one or two sentences) for NPCs here, here are some sample names, here's a D20 encounter table'. Other areas just get two pages.

As you might expect given that, it's completely system agnostic. There are three pages of thoughts on using it in a 5E D&D campaign, but even most of that is applicable to any system.

I'd play in one of his campaigns, and I'd love to see a studio licence it to produce a video RPG, but I don't think I will ever use it myself.