From what I can see none of his own books are actually about "Agile". In fact "Agile development with Rails" is probably the only one that used that word in the title, and I can't really fault him for taking advantage.
Also he wrote some quite good books, 'Pragmatic Thinking and Learning' come to mind. I don't think your comment is particularly fair, nor does it diminish the point he is making in any way even if you're right.
"Pragmatic Thinking and Learning" was Andy Hunt, but as he and Dave Thomas are kind of a double act I can see why you'd get mixed up.
As to your point about whether they wrote any books on 'agile' I think it could be argued either way. More specifically, whether or not "The Pragmatic Programmer" (the book they made their name with) counts as a book about 'agile' is an interesting question, and one that throws some light on the wider debate.
Although they sometimes talk about "pragmatic programming" as if it were a thing, in the same way as XP and Scrum, their approach was more one of giving a smorgasbord of potentially useful techniques and practices rather than codifying a tightly defined set of best practices.
"Once the Manifesto became popular, the word agile became a magnet for anyone with points to espouse, hours to bill, or products to sell. It became a marketing term, co-opted to improve sales in the same way that words such as eco and natural are. A word that is abused in this way becomes useless—it stops having meaning as it transitions into a brand."
No not really. None of those are books he wrote himself. He runs a book publishing company. It's primary purpose is to publish books people want to buy. Though I'll agree that many of those authors capitalize on the exact things he is railing against in his post.
Also he wrote some quite good books, 'Pragmatic Thinking and Learning' come to mind. I don't think your comment is particularly fair, nor does it diminish the point he is making in any way even if you're right.