The best book on application development with Ruby and FXRuby is FXRuby: Create Lean and Mean GUIs with Ruby, by Lyle Johnson, published by the Pragmatic Programmers. The following Ruby books also contain sections on FXRuby:
The Ruby Way, by Hal Fulton.
Ruby Developer's Guide, by Michael Neumann, Robert Feldt and Lyle Johnson.
The current FXRuby API Reference Documentation is a work in progress, but serves as a pretty good guide to the available methods and attributes for the different classes. The regular FOX API Reference Documentation can be used as a fallback source of information, although it is intended for users of the C++ library.
The FOX toolkit home page is still one of the best places to find information about GUI programming with FOX. This is the site maintained by Jeroen van der Zijp, the creator and primary developer of FOX, and serves as the official download site for the FOX library. In addition to a number of articles about programming with FOX, you'll find links to projects using FOX. A newer web site, the FOX Community Wiki, is maintained by Sander Jansen. Like most Wiki sites, the information here is a little more scattered but tends to be more up-to-date than some information on the main FOX site. This site includes a number of tutorial articles, as well as "cookbook" and "how-to" pages on many topics.
One or more of the following mailing lists may be of interest:
The firstname.lastname@example.org mailing list is a very low-volume list for FXRuby-related announcements. To subscribe to this list, follow the instructions at http://rubyforge.org/mailman/listinfo/fxruby-announce
The email@example.com mailing list is a higher-volume list for FXRuby-related discussions. To subscribe to this list, follow the instructions at http://rubyforge.org/mailman/listinfo/fxruby-users
The firstname.lastname@example.org mailing list is a very low-volume list for FOX-related announcements. To subscribe to this list, follow the instructions at http://lists.sourceforge.net/lists/listinfo/foxgui-announce
The email@example.com mailing list is a higher-volume list for FOX-related discussions. To subscribe to this list, follow the instructions at http://lists.sourceforge.net/lists/listinfo/foxgui-users
The Ruby-Talk mailing list (or its mirror, the comp.lang.ruby newsgroup) is a high-volume list for Ruby-related discussions. To subscribe to this list, follow the instructions at http://www.ruby-lang.org/en/community/mailing-lists/