First of all, I would like commend Mr. Clark for addressing this topic at all. Accessibility is an important in computer development.
While the content seems to be accurate, and quite detailed to the point that you could use the information in the book to actually build a site with it, the writing is so poor and very difficult to read. Mr. Clark needs to throw away his thesaurus and hire an editor. He would be better off delivering his message in a clear and concise manner, and spend less time writing in a very "fancy" way that would be better suited for thesis papers rather than a book targeted towards the masses. This heavy style of writing makes it a difficult book to digest while riding the subway.
Pretentious - the one word I would use to describe the overall style of writing. This really put me off as a reader, making the content even more difficult to absorb. (In one case, the author actually explains his choice of word, "indention" as opposed to the more commonly used "indentation". His explanation seemed to imply that everyone who uses "indentation" is clearly *wrong*; two online dictionaries confirmed that "indentation" could have been used.) Of course, my opinion that the writing of this book is pretentious could also come from actually having met the author.
Read this book only if you really have the time to struggle through all his big words to get at the meaning behind them.