This is a book for people who want to build an advanced social network site.
Pros:
-covers many aspects of SN sites, such mashups, mobile interface, etc. very comprehensive.
- clean codes, relatively few errors
Cons:
- not for beginners
- teaching style could be better
Overall I feel the author is a seasoned web developer, but may not be the best instructor out there. An obviouse comparison can be made between this and Railsspace, which is on the other end of the spectrum: less coverage but somewhat more in-depth discussions. So, if spending some extra bucks is not problem for you, I recommend buying both.

a decent book for beginners. Very good tutorial style. If you are new to RoR and have interests in building social networking sites, you may want to take a look at this book. However, personally I think its coverage is quite limited