It's funny that Williams quotes H. L. Mencken's remark that most books about writing are badly written. He first quotes it, then goes on to prove it.
Normal humans from Planet Earth wouldn't say "stylistic infelicity" when they meant "bad writing". They wouldn't say "peripherally relevant" when they meant "closely related". And they wouldn't dream of saying "topicalize X", not even under torture, if what they wanted to say was "make X the topic of the sentence". (You read that right, the guy unashamedly says "topicalize".)
Want some idea of what you'll be getting yourself into? Check out this boner of a sentence, typical of the writing style of the whole book:
"But the object of our attention is writing whose success we measure not primarily by the pleasure we derive from it, but by how well it does a job of work."
Someone ought to tell this guy to omit needless words. The parallelism isn't parallel, the phrase "of our attention" is pointless, the phrase "whose success we measure" is awkward, and that "job OF WORK" is simply nauseating. An Earthling would write something like this:
"Our goal is not just pleasant prose, but effective prose."
So the whole book is written in turgid-ese, even while trying to speak out against it. It's all just an endless wearying slog through the mire. Not unintelligible, just not worth the effort. For what do we learn at the end of the Long March? We learn we should omit needless words.
Last but not least, the book is a typographical disaster, with everything jumbled together and packed into the page. Skimming is impossible.
Many of the five star reviews here are from technical writers, engineers, and so forth. I see a guy from MIT, another from Compuserve, and that's as it should be. They're enured to bad English already, and I'm sure that compared to an engineering textbook this is John friggin' Keats. But for the rest of us, it's just not good enough.
(It's by a linguist, after all, and what the heck do they know about language?)
So it's back to Strunk and White for non-fiction. If you're interested in clearing up confusion in your fiction, check out "Writing and Selling Your Novel" by Jack Bickham, especially chapters 4 and 6. Teachers should consider "Classical Rhetoric for the Modern Student".
However, I found Williams' writing to be confused. At the beginning of one "lesson" early in the book he claims to not want to explore writing and grammar in terms of "rules", but rather principles, yet 3 pages later (and throughout the rest of the book) he belabours us with rules and check marks next to "good" sentences. His attempt at an artistic, ethereal exploration of style only reveals and emphasizes moreso that there are rules of writing and grammar. I found "Technical Writing Style" (Dan Jones) a much better text with comprehensive exercises for mastering style (with any prose writing (technical or not).