Again, a bit of a bucket... things just get thrown in as they arrive...
http://en.wikipedia.org/wiki/ABC_notation Wikipedia article (maybe add some of these links to it)
http://abc.sourceforge.net/standard/abc2-draft.html
http://abcwiki.selfthinker.org/abc:syntax very interesting site, hosting a wiki dedicated to ABC notation among others
http://abcnotation.com/wiki/abc:standard:v2.1 another wiki, or an alias?
http://abcnotation.com/blog/2010/01/31/how-to-understand-abc-the-basics/
http://abcnotation.com/blog/2009/12/23/how-to-get-started-with-abc-notation/
http://mudcat.org/thread.cfm?threadid=121902
http://trillian.mit.edu/~jc/doc/doc/ABCtut_ABClinks.html
http://trillian.mit.edu/~jc/doc/doc/abc-15.txt
http://trillian.mit.edu/~jc/doc/doc/ABCprimer.html
http://www.norbeck.nu/abc/bnf/abc20bnf.htm Backus-Naur form v2.0
http://www.norbeck.nu/abc/abcbnf.htm v1.6