The linguistic part of things is interesting, but the author branches out into a wide range of other subjects, about which he knows no more than any other not-very-inquisitive layman. I was particularly galled by his confidently repeating the insulting myth that Polish cavalry charged German tanks and were slaughtered. It was 2012 when this book came out, for Heaven's sake! He had no excuse for not having known the truth, which is very easy to learn these days -- it's even on Wikipedia, in five or six different places.
His cavalier approach to history (in fact, we might say that he's confidently charging the tanks of true information and being slaughtered...) leaves me with every reason to suspect that he's equally cavalier about everything else outside of his specialty -- which is to say, at least half of the book, including many subjects where I'm not knowledgeable enough to distinguish truth from nonsense. For that matter, how do I know that he's not repeating the trite nostrums of fifty years ago _within_ his specialty? I'm not a linguist, the closest I come is reading the Language Log.
Jacques Barzun had nasty things to say about people who see inaccuracies in minor things as proving inaccuracies in major things, but I doubt that that's what I'm doing here. The horses-charging-tanks myth is seductive (to people with a certain outlook on life), widespread, and profoundly wrong; by repeating it, the author has proven that he doesn't have a good eye for the difference between the truth on the one hand, and plausible baloney on the other. That's a pretty dangerous trait to have, if you're undertaking to teach your readers about the patterns of history and how civilizations rise and fall.