Ruby Annotation Recommendation http://www.w3.org/TR/ruby

CSS3 Ruby Module http://www.w3.org/TR/css3-ruby

W3C i18n test for ruby markup http://www.w3.org/International/tests/#ruby

Ruby Annotation in XHTML 1.1 http://www.w3.org/TR/2001/REC-xhtml11-20010531/doctype.html#s_doctype

Implementing the Ruby Module http://www.w3.org/People/mimasa/test/schemas/NOTE-ruby-implementation

There is useful information about ruby in Ken Lunde's book, CJKV Information Processing (ISBN 1-56592-224-7), especially chapters 6 and 7.