They used edge rails so they could write about it before it was officially released. Here's an excerpt from the book: "However, edge Rails still comes in handy

They used edge rails so they could write about it before it was officially released. Here's an excerpt from the book: "However, edge Rails still comes in handy sometimes. For example, it allowed us to extensively cover Rails 2.0 features in this book prior to the actual, official 2.0 release"