One thing that I have found about books that are written by linguists (on whatever topic) is that they tend to be just a bit too long. (I have John McWhorter in mind. I've read two of his books-- Losing the Race: Self-Sabotage in Black America and Doing Our Own Thing: The Degradation Of Language And Music And Why We Should, Like, Care) and have found that they were at least twice as long as they needed to be.
1. If this book had been permitted to be expanded into a full length book, it would have joined the pantheon of Bloviating Books Written By Linguists. Even as I read it, I could see where the editor had to go through with his/ her blue pen and cut out a lot of the waffle. When the author started with his biological metaphors, he really did get quite wordy and a lot of what he said could have been truncated to 2 or 3 paragraphs.
2. Since the author used biological metaphors (with which he went WAY TOO FAR), it made me wonder: "Is there even an issue here?" We all know that more species have existed and have died out than currently exist (1) and that it is perfectly natural that species died out in the churn of evolution (2). So if these same things happen in the context of languages, then *so what*?
3. The author talked about the loss of knowledge that goes with the extinction of languages. But then the question comes up: "If 50% of languages are on their way out, does that mean that 50% of knowledge is on the way out with them?" Also: If knowledge is really good and useful, then it does not depend on language. Did it matter that Il Principia was written in Latin and that no one speaks it anymore? The concepts have been translated into as many different languages as are useful.
4. The author also has a very rosy picture of multilingualism (in a book that I thought was supposed to be about some of the technical details of languages). He gives examples of where groups of people can get along speaking multiple different languages, but it's *not always that easy*. (French Canada. Africa. Sri Lanka [!]). And what was the point of getting into a series of value judgments in a book that was supposed to be descriptive (and not prescriptive)?
5. The book fell apart toward the end. It just got.....babbly. About midway through, the author promised to take up the age-old "Is Chinese a language or a dialect?" question at some length, but he just didn't. His discussion of "internal" vs. "external" concepts of a language was not very clear. The book could have lost about 25% of its length with no diminishment.
In spite of all that, there were some good take away messages.
1. We got an idea of how many languages there are in the world--along with some slightly strained analogies to Biology. (6,909 is a reasonable working number.)
2. We got a taste of the techniques that are used in studying the divergence of languages.
3. There were some answers as to why languages diverge over time-- no matter how much effort any academy/ government might make to keep them intact.
4. 25% of the 6909 languages have less than 1,000 speakers and over half of them are headed for very likely extinction within the century.
5. We were introduced to some basic concepts, such as "phonology"/"morphology"/"syntax."
Verdict: (Barely) worth the time (3 hours/ 1-2 afternoons) and worth the money ($6).