From the very first page, the author of this book lets us know exactly its purpose - to act as an off-line reference to the hundred or so domestic PHP functions one might use in the course of writing software, and a couple of their lesser-used variants.
One of my favorite parts was the list of related functions below each description: this allows you to hyperlink your way around the book in a more semantic way than simply reading it front-to-back.
This is certainly not a book just for beginners, because I wouldn't consider output buffer functions to be used by beginners (if you don't know what I mean when I say ob_start() or ob_flush(), then you should probably pick up a copy).
This is the kind of book you should throw in your laptop bag as you get started on a new project, or keep on the shelf of your cubicle. Think about which is faster - tabbing back and forth between your browser window with the current page you're testing, and your text editor, AND the php.net site, or just having a small reference book next to your keyboard.
Also consider your ability to make notes next to functions about your own personal tricks and experiences - I always find myself digging back into my own code as a reference, but if you just had this paper version to carry with you everywhere, you could save yourself a lot of time by not having to grep through your code.