See also: Run Junkie's Your First 50 Miler
The April issue of Runner's World has a nice collection of articles for runners looking to see what's on the other side of 26.2 miles. Meant for the newbie but an interesting read for others as well, the collection includes a 50 mile training plan (link), a primer on surviving your first ultra with minimal effort (link), and, of course, a number of stories of personal triumph. The articles seem a bit scattered online. For a nice soup -to-nuts read, you may want to grab the hard copy (on newsstands now!).