This is probably the best overall book you will find about guitars. It covers quite a bit of material. The details about how a guitar works and how to maintain a guitar are the best I have ever personally read. For example, setting the scale length of a guitar, a very important part of getting a guitar to sound good, is covered in easy to understand terms. It helped me fix some minor tuning issues on my Stratocaster.
This is also an effective book if you are interested in buying a guitar. You will be able to identify problems with most guitars because you will know what to look for. For example, checking for a warped neck. There is a great two page section that gives you the basics about how to look for that kind of defect.
And don't let any of these other reviews convince you that there is not enough theory in here. Granted, if you are brand new to playing or music in general it probably won't make much sense and you will struggle. But if you understand some basic scale and chord structures this book will help you gain a better understanding of how it all works. Odds are that those who do not like the theory in this book either do not like the approach or didn't actually read the entire book.
This book will NOT make you sound like your favorite guitarist. All the great players are great because they practice and the greatest players just have the gift. So do not expect that out of any book or instructor.
Lastly, do not buy the hard cover edition. Get the soft cover/paperback edition. You will want to have this book in front of you when you are working on your guitar or playing it. The only thing that would make this book a 5 star is if you could get it in spiral bound format.