BackpackingLight.com. In 2000 this was the best site for people who wanted information about light weight backpacking. In the last few years I feel like they have been stumbling a bit (ugly UI / too many ads, etc) as they have tried to figur out how to be profitable. This site has a mix of articles that are free and availible to a paid subscription, and a forum community which is free. This is one of the best places to learn about what is on the cutting edge of backpacking, be in new materials, gear, or technique. The subscription is worth it if you are in the process of trying to lighten your load.
Hiking and Backpacking by Buck Tilton. Part of the Knack: Make it Easy series. Easy to follow with plenty of pictures. I tend to go a bit lighter weight than what this book discussions, but it’s a good, gentle introduction.
The Backpacker’s Handbook by Chris Townsend (now in 4rd edition) is the best general book I have found. Balanced and informative without being as voluminous as The Complete Walker. I wish I could find a shorter book that I could recommend.
Allen & Mike’s Really Cool Backpackin’ Book by Allen O’Bannon is a humorous introduction to backpacking with funny illustrations. I personally found Chris Townsend’s books more useful that this. I thought Townsend’s Pocket was easier and clearer to read, and his Backpacker’s Handbook has a much richer discussion about gear selection.
Lighten Up! Complete Ultralight Backpacking by Don Ladigin is the best book to help someone who is a traditional backpack thing about lightening their load. If I could write well, this is exactly the sort of book I would hope to write. Rather than getting bogged down in specific products which would become dated, Don looks at general principles which are easy to apply.
The Complete Walker IV by Colin Fletcher is the classic book, comprehensive, through. Great discussions about techinque. Interesting discussions about gear. My only regret is that ultra-light gear (other than tarps) are not covered well.
Mountaineering: The Freedom of the Hills edited by Steven M. Cox, Kris Fulsaas is the classic guide to mountaineering. Pack full with useful information about mountain environments, snow techniques etc.