Any book that claims to cater for beginners and advanced users alike has quite a task ahead of it and I don't believe this guide to LaTeX does justice to either category of user.
The true nature of this 616 page beast is spelt out in chapter 1, where the reader is advised that the book "is designed for LaTeX users who have little or no experience with computers" and that there exists "considerable repetition in the text". Unless you have a solid week to spare and the memory of a gold fish this book has the potential to be incredibly frustrating. Instead of being immersed in worthwhile examples demonstrating the true power of LaTeX, the reader is forced to trawl through paragraph after paragraph of verbose explanation. Worse still, with minimal imagination employed in presentation, the fact that this book was typeset using LaTeX doesn't inspire confidence - helpful hints supposedly written in a smaller typeface to make them distinct, simply disappear into the sea of sentences.
With a sales rank of 5046, this book is by some margin the best selling reference for LaTeX, out selling texts by Goossens et al (rank 14628, 400 pages, published 1994) and LaTeX developer Lamport (rank 33235, 272 pages, published 1994). This is because, as other reviewers have correctly pointed out, this book covers more than the others combined. The plethora of appendices is dense but in some cases not as useful as would first appear. For instance, one table included contains a complete list of possible PostScript fonts, great you say, until you notice that they're all displayed in the same font! However, combining this unrivalled brevity with the fact that LaTeX is updated on an annual basis and it's not hard to see why you might opt for a book published in 1999, which is larger and cheaper than both alternative books published in 1994.
With a LaTeX 3 version in the pipeline and with the internet offering a number of compact LaTeX HOWTO's and quick references (most notably "The Not-So-Short Guide to LaTeX"), a wise alternative might be to arm yourself with a downloaded guide and actually learn through trying (inevitably making your fair share of mistakes along the way) and to hold off to see what the next generation of published LaTeX references have to offer.