The FIT for Developing Software: Framework for Integrated Tests (2.4 MB) free eBook by Rick Mugridge and Ward Cunningham is available for download.

"This book is about using Fit tables for two major, interconnected tasks:

• Helping to think about and communicate what is needed in a software application
  through concrete examples.
• Automatically testing that the application is doing what is expected, from a
  business perspective, and that the application continues to do so as it grows
  in functionality.

These two tasks are essential to the value and quality of a software application.
But they are often carried out poorly, leading to breakdowns in each step along the
way from business need to a running application."

[via Clarke Ching's blog]