Beyond Backpacking is something every hiker should read. I don't agree with everything Jardine says but that book changed how I look at my pack.
The Complete Walker was another good one.
And of course Ed's Hammock Camping book. All three already listed!
I read all kinds of trail journal style books, too...mostly all interesting but nothing was "must read" except for Bill Bryson's piece of half-fiction. I laughed out loud so many times that I don't care whether he finished or not.
Bookmarks