If, like me, you want this book for self study, then whether it will be useful for you depends on your preferred style of study. For me, the book was dissapointing when it arrived because it felt like it was not really suitable for self study. The introduction to the book made it very clear that the primary aim of the book was for it to be used in teacher training classrooms, with usefulness for self study as a secondary aim.
The book doesn't contain chunks of information 'about language'. Rather it has a list of 'tasks' for each unit and then, in the back, answers for the tasks. For example, in the tasks section one task will ask you to match terms and thier definitions, and the 'answers' section in the back will have a completed table. This can make it cumbersome to use for self study if you don't already know most of the terms, because you end up studying the answers page and then returning to the tasks page. So, you end up flicking endlessly between the back and front, compiling information by finding answers to questions rather than just reading it. For some people this may be a useful method, but for me it just felt awkward. Evidently, the tasks are designed primarily for classroom use (although all can be completed alone).
'About Language' does cover a lot of key elements of the language though and, after I did wade through it, it did help me understand the language a lot better. Now I know, for example, what a bilabial fricative consonant is, and where it is on the phonemic chart!
In summary - this can be a useful text but bear in mind that the approach is 100% task based which (if like me you didn't pick up on the hint in the subtitle!) may not be exactly what you thought it would be.