Mark Sobell is a class act. He has been a mitzvah to both the programming and writing sides of the *nix equation for 30 years. Free is about $50 less than this download should cost.
The only real basis for my opinion is seeing .edu somewhere in the URL when these things are posted. So, I could be wrong, but I can remember two other recent examples: an HBS case, and a couple of chapters from Jared Dimond's Guns, Germs and Steel.