Online Books
Programming Ruby (Pickaxe)
The definitive Ruby reference by Dave Thomas & Andy Hunt, available to read online. Also known as the Pick Axe Book. Also available via Ruby Central.
Sean Chittenden's rubydoc
An open-source Ruby reference written collaboratively by various members of the Ruby community.
API References
Standard Library API
Basic documentation for the classes, modules, and libraries included with the standard Ruby distribution.
Tutorials & Translations
Einführung in Ruby
A German translation of Daniel Carrera's "Introduction to Ruby", translated by Robert Gogolok. Chapters added as they are completed.
Introduction to Ruby
Daniel Carrera's introductory guide to the Ruby programming language. The source material for the German translation hosted here.
Documentation Tools
RDoc
The essential Ruby documentation tool. Creates documentation in HTML, XML, RDF, and other formats by parsing Ruby source code and embedded comments.
Rimport
Allows creation of ri data files from RDoc XML output. Useful for importing new data files into the ri documentation system.
RUDE4C
Generates documentation from Ruby library extensions written in C. Scans code for expressions like rb_define_class to extract class and module trees. Outputs plain text, rd-style, or HTML.