The book FXRuby: Create Lean and Mean GUIs with Ruby (available from the Pragmatic Programmers and other booksellers) is the best print source of documentation for FXRuby. It’s also available as a PDF.
Several other books, including Ruby Developer’s Guide and The Ruby Way, include chapters or sections on GUI development with FXRuby.
The FXRuby User’s Guide provides detailed installation instructions as well as a handful of tutorial exercises to get you started with FXRuby development.
If you’re looking for API reference documentation, all of the classes and methods for FXRuby are documented here.