Book recommendation is the John Muir Trail by Elizabeth Wenk with Kathy Morey, Wilderness Press, 4th edition, plus get her spreadsheet of GPS waypoints ( http://www.whitneyportalstore.com/data/EWenk_JMT.htm ).
Some good links:
I'm doing a 161 mile segment from Reds Meadows to Whitney Portal in late July so I can't speak firsthand about the trail except the segment from Wallace Creek to Whitney Portal which coincides with High Sierra Trail which I've done. I'm sure others (such as Bob B.) will pipe in with suggestions and they know more firsthand about the trail.
Definitely, join the John Muir Trail Yahoo Group (a lot of BPLs are on that list too):