Short story: If you are fresh to social networks, this book will get you up and moving. It explains how to build a fully featured social network, based on a custom underlying framework. At the end of the book you will have a functional site, and know the what’s what of social networks. It’s very easy to follow the author, and the code is very readable. New concepts and features are introduced in a way that makes it easy for the reader to keep up.
PS: Packt currently has a 40% off special offer on this book, so I recommend you pick it up via their website!
Slightly longer story: I must admit that I’m having problems seeing the value of rolling your own MVC framework when building applications like this. The main arguments made in favor of going custom is ease of maintenance, licensing and code efficiency. I really don’t buy these, as there are several good frameworks available under the BSD license, which are proven time upon time (Yii, Zend, Code Igniter, Symfony). Not only is the code maintained by teams of highly skilled developers, but also reviewed for security holes and exploits by the less good-mannered of our beloved internet. A *very* important aspect of social networking sites is security, and I feel that the book should spend more time on that subject (maybe including info on cross site scripting and related).
However, the one argument I really do see the value in is the process of learning. Writing a PHP framework is a very valuable exercise, and if you still have that on your TODO-list, this book should serve as a good guide on the way. The framework produced will allow you to keep extending and improving features after you are done with the material covered in the book.