I've found internal documents written by smart people and smart companies to be infinitely better than stuff put out there for PR which tends to be weighted-down, caveated, and also more basic than the hard-hitting internal stuff.
Two of my favorites --
1. Ray Dalio's "Principles" -- from Ray Dalio, the brilliant manager of the world's largest hedge fund. His "Principles" is one of the most important and valuable reads I've found:
2. Valve's "New Employee Handbook" -- a great document on culture that provides a look at how a sharp non-traditional company does it. Quite good:
I've also found a lot of value in documents released and/or declassified by the U.S. military and intelligence services. For instance, the "Warfighting" from the U.S. Marine Corps has a lot of really clear thought and valuable lessons in it:
Got any favorites you've come across throughout the years?