The SAX specification is defined at http://www.megginson.com/SAX/.
James Clark's expat is an XML parser written in C. is the golden standard for XML parsers. It is the library upon which the Ruby standard library's xmlparser.rb is built.
XMLParser by Yoshida Masato is a Ruby library built on top of expat. It is available at the Ruby Application Archive.
Programming Ruby, The Pragmatic Programmer's Guide, by David Thomas and Andrew Hunt, is a well-written and practical introduction to Ruby. Its Web page also contains a wealth of Ruby information. Though the book is available online, I encourage you to purchase a copy.
Jonathan Conway has kindly started to mirror the NQXML .tar.gz files at http://www.ugcs.caltech.edu/~rise/nqxml-mirror/.
Have I mentioned the NQXML home page yet? I have? Never mind.