The is essentially a programming cook book with recipes centered on social networking functionality. That being said - this is not a "getting started on rails" book; you should have some idea of how to use rails (but you dont have to be an expert).

Here are some of the recipes: developing a CMS, blog with RSS, blog with web services, a forum, photo gallery (probably would have been better if this one had used Amazon's S3), adding tag functionality, creating a mobile version of your site, XFN, ...

Unless you're a broke student debating between this and food, I'd get this book. There are decent examples that are useful to see. It's pretty good considering Amazon's cheap price and considering this is the only book out on Rails related to social networking sites (as of when I wrote this review).

Update: I've now read the other Ruby on Rails Social Network Book: RailsSpace. In my opinion I would get both. However if you have to choose one it would depend both on your skill level and taste. Would you rather have more subjects covered with less material, or would you like less subjects being covered more in depth. If you want more subjects covered and you think you can figure out the extra details, then Practical Rails Social Networking Sites is for you.

The book is interesting, but has quickly become outdated. While it covers the basics, it is vague on specifics. To me, a social networking site is something like MySpace, and the book does not go into the depth I'd like to show me how to make that type of site.

Also, Ruby on Rails is far from an enterprise platform. There are also many social networking software packages out there that are a good start out of the box.