I bought it a few weeks ago. Impressions are twofold. On the one hand, there is useful and interesting information, besides, in addition to the book, he gives several mini-lessons by e-mail. On the other hand, a somewhat peculiar presentation of material ... Although the latter can be my subjective opinion. In any case, I believe that its $ 20 it is probably worth it. At least I did not feel disappointed.
PS. This is really introduction. Do not expect a full course of study.