the guidebook is randy vogel's "joshua tree rock climbing guide". chockstone press, evergreen, colorado. someone above says it's out of print, but i don't know about that -- i have the '92 edition. it's extensive and excellent, w/ a few thousand routes, maps of the rocks, and many many photos of routes. i say buy it if you can afford it, it's very useful for routefinding and also very fun to just page through.
i was at j tree a few weeks ago, camped at ryan. it's awesome, and crawling with climbers this time of year. it gets cold at night (30F), and to reiterate what others said, no water or firewood in the park, so buy it on the way in.