The two interesting bits of Wolfenstein 3D everyone always talks about when they talk about Wolfenstein 3D are the ray-casting algorithm and the LFSR powering the FizzleFade that happens when you die or kill a boss—the rest of it is mostly just predictable boilerplate and unexciting optimisations (some of which may be of interest to VGA historians, but almost all of which will certainly do more harm than good today), plus one or two semi-interesting bits about AI that turn out to be very rudimentary. Because those optimisations and some of the other choices require some techno-historical context, Sanglard also includes a bunch of that (some of it is interesting (I want a Disney Sound Source now), a lot of it is very much not), and because nothing in video games can ever not be about Personalities, there are a lot words about the people of id Software as well (Sanglard's admission that you're better off reading Masters of Doom if you want that notwithstanding).
Regardless, the stars of the show are arguably still those two algorithms. If you're hoping a self-described engineering book would give you a rock-solid explanation of them, though, you're out of luck: while Sanglard's treatment of ray-casting is barely adequate (just barely: no mention is made of a camera plane, so if you follow along with the algorithm he actually describes you end up rendering a vertical line), that of FizzleFade is hardly there at all—Sanglard mentions how to implement a linear-feedback shift register, but doesn't seem to know why it actually works. Yes, it's somewhat esoteric and math-heavy, but that's why I bought a book instead of reading a blog post.
(And actually, there are two more non-trivial algorithms it would have been good to hear more about: assets are compressed using Huffman trees and what John Carmack claims is an independent discovery of LZSS (Sanglard himself calls it Lempel-Ziv, but that's a broader class of algorithms), but apparently neither merits more than a name-drop.
Meanwhile, fixed-point arithmetic gets half a dozen pages of laborious schematics. What Sagnlard considers to be self-evident and what he considers to be black magic meriting great elaboration is... not always easy to agree with.)
Other, more minor quibbles I have with the book include much being made of VGA mode 13h's non-square pixels but most of the printed sprites and textures (and, I think, some of the screenshots) having square pixels, typos and grammatical errors still being fairly common despite this being the second edition, and the horrible covers that curl up as soon as you look at them and whose matte finish will leave you with a Game Engine Grimy Book before you make it out of the introduction.
All in all, Game Engine Black Book: Wolfenstein 3D really feels like a self-published book, and it could have benefited from a (relatively) lay editor going over things for clarity and structure. It's also the only book that does what it does for Wolfenstein, though, and, for all its flaws, if that game was as big a part of your life in the early '90s as it was of mine, you'll find it a serviceable nostalgia piece.