One "problem", aside from everything that has already been mentioned, is the book is not as high quality as it could be if it would have gone through a peer review process. There are problems with the shading and with the AA in the example pages. Of course that is easy to correct in a digital medium and the author might want to do so too but given the bigger problem of plagiarism/theft that hangs in the air, there probably is not much point to do that anymore.
Now that we seem to be also discussing books/knowledge in general and not just this book/knowledge in particular, one thing about books (granted, this is less of a problem with digital distribution) is that the information in them once written remains pretty static and immutable for long periods of time (until revised copies are released) while the medium and the community is constantly changing and evolving.
I think there is more to learn from an active community with "fresh" information, interaction and by actually doing stuff than by merely reading books because of the one-directional nature of information flow between author and reader.
So well, the plagiarism thing aside, ... forums, websites and free tutorials are doing a better job at teaching the art than that book and because of being free are also more welcoming to newbies who want to learn about it.