If you compare this book to most of the other books on PHP, you'll quickly notice that this book is much smaller than those thousand-page behemoths. The relative smallness of this book is intentional-I've found it's easier to learn from a book you can actually hold!
That being said, plenty of topics that are covered in those lengthier books are not covered in this book. This book offers, as its title, says the essential information. In other words, information that will provide a solid foundation for the additional topics you will find in longer, more advanced books.
A main characteristic of this book, besides teaching the fundamentals, is that you don't have to be a computer scientist or programmer to learn from it. If you are a computer scientist or programmer already, you may not like the rather prosaic nature of the explanations and instructions in this book. Ultimately, I wanted to write a book that someone could take off the shelf, skim through, and say, "Hey, this PHP thing looks like a neat language, and ever-so-easy to learn!" Because it is!
If you've been programming with PHP since the beginning of time, there's probably not much you can get out of this book, except to hand it to your boss and say, "Look! Another book on what a wonderful language this is. Can we please stop using ASP/Cold Fusion/Java/Perl/C++ and migrate to PHP?" But if you've just dabbled with PHP or have never seen a PHP script, I am certain this book will be of some use to you.
Additional information for this book (and for other books I've written) can be found at http://www.thickbook.com/. At this site you can download all the code samples in this book, as well additional tutorials and notifications of any errors in the book. You can also use the site to get help with any problems you may have with the examples.