I consider this 2 things at once. It's a stand-alone rules system that's heavily inspired by B/X rules, and it's also a giant pile of rules for downtime, high level adventuring (you're a king with a keep now, etc), running campaigns, extra proficiencies, henchmen/hireling/experts hiring, etc. So I'll write two separate reviews here:

High level play, campaigns, stronghelds, NPCs, etc SOLID 5 stars. One of the best sourcebooks I've ever seen. Straightforward and useful. Many products say "here's the price of a tower for your keep" but this gives you entire frameworks for actually gaming as somebody who has a stronghold. Many products have some simple stuff about hiring experts, this one goes into lots of excellent detail. Tons of proficiencies, getting lost, foraging, wilderness rules, special maneuvers, sea combat, drowning, crimes and punishment, trading, plant toxins, etc. This section is absolutely worth the price of admission and I recommend the book for this. I see this book as all these wonderful things to drop into my campaign with my rules system.

Stand-alone B/X-inspired rules system 2 stars. Basically trash. It's as if people took OD&D and make a couple completely arbitrary changes that make it less compatible and more mathy without any actual gain. Don't buy the book for this, buy the book for the stuff in the previous section which is fantastic.