I can't say enough good things about this book. Being a big .NET guy, I decided it was time to venture outside the Microsoft stack just to see what else is out there. After all, I just can't stop learning; what fun would that be? Well I didn't expect it happening, but I fell in love with Ruby on Rails. You can read more about my "affair with RoR" here: http://bit.ly/bfEkgT.
Agile Web Development with Rails is simply fantastic. It's loaded with great content. To me, this is "the" Rails book. I'm really looking forward to the 4th release (coming soon), which will cover Rails 3.