Untill the time I`ve written the essay in English about this theme ( sorry, at the moment it is just available in German Language ) I would like to recommend some websites instead of:
http://nbthk.net/NBTHK/Japanische%20Schwerter.html
http://nbthk.net/NBTHK/NBTHK%20Startseite.html
http://www.shikokuhenrotrail.com/japanhistory.html
http://en.wikipedia.org/wiki/Category:Japanese_eras