Unfortunately it doesn’t seem like the new support for Engines in Rails 3 is in the updated book. Many developers today take advantage of multiple Rails applications coordinating together into a single service. For example, looking at the URLs here at PragProg, the forums are a separate Rails app (Beast) from the Store.
Explaining this concept and showing how to integrate separate apps (or use Engines) would be an excellent topic for the updated edition.
Since that may not happen, does anyone know of good resources for writing Rails 3 Engines and/or integrating multiple Rails apps into a single service?