Peter Armstrong has just released the initial version of his PDF-only book, Flexible Rails. It's available for $20, and anyone who buys the book now will get free PDF copies of every subsequent version of the first edition of the book. You can also order the book in 5 copy, 10 copy, and 30 copy editions for use in the workplace.

The book is at 214 pages and Peter claims is only 20-40% complete so far. Despite this, the current table of contents includes:

Overviews of Rails 1.1, Flex 2, Flash 9, and how they all work together
How to install everything
Creating an initial 'hello world' project
Flex login systems
Sending XML with HTTPService
Creating a basic GUI with MXML
Getting tasks to show up in the task grid
Debugging XML
Refactoring

Looks like it could become the guide for those interested in the interoperability of Rails and Flex. Check it out.