Books: “Cross-country Skiing: building skills for fun and fitness” by Steve Hindman; The Mountaineers, 2005. Comprehensive treatment of all aspects of cross-country from racing to backcountry travel. The descriptions of basic and advanced technique are excellent. If you want just one book on cross-country skiing, this is it.
Videos: Thousands of cross-country skiing videos are posted on youtube, vimeo and other sites. These selections are among the best at explaining and demonstrating basic technique.
Classic (Diagonal Stride) Technique:
Free Style (Skate) Technique: