Kirsten Flagstad (1895-1962) - Opera Shop page offers photo, brief biography, and audio of Isolde's Liebestod. - http://www.bassocantante.com/opera/flagstad.html
Wikipedia: Kirsten Flagstad - Detailed biography including links to representative recordings and a biography published in 1982. - http://en.wikipedia.org/wiki/Kirsten_Flagstad
Kirsten Flagstad, The Voice of the Century - Museum in Hamar, Norway houses a collection of costumes and photographs from the soprano, with online image gallery, discography, and operatic roles by year. - http://www.kirsten-flagstad.no/