'Language Death' does a hard job well. The author seeks to show both the need for language preservation, and at the same time provides an overview of the process by which language preservation can be done.
The book started slowly for me: the first section is an argument in favor of language preservation, and a discussion of language death, and I found the arguments in favor of preservation to be a bit long and over-drawn. But then, I didn't need to be persuaded; I think language diversity is a good thing, and those not yet so convinced may need more work. But the book is overall well-done, well-written, and concise, and entertaining and thought-provoking as well.
An earlier reviewer (who's also a buddy of mine) suggested that the book gives insufficient credit to Bible translators in the job of language preservation. I'd suggest that Crystal may have a slight bias against Bible translators, especially when he refers to the work done by Bible translators as being biased. I might prefer describing it as narrowly-drawn, rather than biased.
But having said all this, the book handles a tough task in a easy to read manner, and gives a good introduction.