An on-line history of music printing:
http://www.musicprintinghistory.org/

A video about music engraving from G. Henle:
http://www.youtube.com/watch?v=345o3Wu95Qo

Another video about music engraving from G. Henle:
http://www.youtube.com/watch?v=345o3Wu95Qo

A video about the Halston process:
http://www.youtube.com/watch?v=VUwu4MOX3UY

Werner-Icking-IMSLP Music Archive:
http://icking-music-archive.org/index.php

Library of Congress Performing arts Resources:
http://www.loc.gov/performingarts/ 

Music Publishers Association:
http://www.mpa.org/

United States Copyright Office (information about copyright, etc.):
http://www.copyright.gov/circs/circ56a.pdf

Music Library Association (information about copyright, etc.):
http://copyright.musiclibraryassoc.org/Resources/FAQ

Proofreading symbols:
http://www.chicagomanualofstyle.org/tools_proof.html

Printing terms:
http://www.printusa.com/glos.htm