Author's note from the book:

"...I discovered Fabien Sanglard’s website and began reading his Game Engine Black Books on Wolfenstein 3D and Doom. Inspired by those works, I wondered whether I could do something similar for Commander Keen: open up the source code, explore the files, and piece together a picture of the overall architecture and the clever tricks used. The style, dimensions, and structure of this book are intentionally similar to Fabien’s Game Engine Black Books, as an homage to those masterpieces. To give it a personal twist, I inverted the title and cover to white."

Wow I assumed it was one of his books before I saw your comment. Thanks for pointing that out!

And his site looks like another thousand, as I'm sure he knows.

We consume his site for the content, not for the minimalistic design that exist since the inception of the universe.