I'm relatively new to Facebook development, and not nearly as fluent with PHP as I'm sure many of the folks considering this book must be. So, you should probably read this review with that in mind.
I bought this and FBML Essentials at the same time, hoping that the two together would provide a sufficient understanding of the api and mark-up language. This book, as described by the other reviewers, has many typographical errors, which of course can be a nightmare if you're relatively low on the learning curve (as I am/was). If you choose to buy this book, you should most certainly download the source code from the wrox site to accompany your reading (as it seems it to be error free and because the code snippets in the book often leave critical methods and files out.)
The greatest shortcoming of this book is its lack of applied examples... many of the more advanced features in later chapters are listed in almost dictionary-style format, with no illustration or example code to explain how the methods and such are actually written and how they might be applied. Again, if you're well acquainted with php, you may be less in the dark than I and thus able to conceptualize how everything fits together. This is definitely NOT a book for novices (I found the O'Reilly book, although short, to be a more effective introduction to development via FBML in this respect (though it doesn't touch much upon the api and other more sophisticated techniques broached in this text.)
That said, the fact that there is such scant information available for Facebook development make this book worth your consideration; but I would definitely consider supplementing it with a book like O'Reilly's, and couple it with a pretty thorough acquaintance with the developer's wiki on Facebook. Even downloading and stepping line-by-line through the footprints application from Facebook will help you get the sense of how this stuff works.
Also be forewarned that this, and most of the books presently available, are not written for the new Facebook API written this summer (2008)... so methods like require_add() are no longer recognized and throw errors... this of course can be quite confusing if you're new to all of this.
Using this book, the wiki, and O'Reilly text, I was able to make my first Facebook App that lets users feature a particular video pertaining to a charitable cause from a small library in their profile boxes and application tabs. If you're trying to do something basic like that, you should be able to do so with these resources. Good luck!