These are all various forms of online Ruby documentation. (By "online," we mean softcopy, not necessarily Web-based.)
ri This command line utility summarizes information about classes, modules, and methods specified as parameters.
myri This is a GTK-based GUI version of the ri utility.
Programming Ruby by Thomas and HuntThis book, the first Ruby book in English, is available on the Web in a downloadable form and as a Windows help file.