We used "Hiking California's Desert Parks" by Bill Cunningham which we purchased on-line used. The new Falcon editions have one book per park but we wanted to only carry one book for the region since we went to multiple parks. We also used the "Trails Illustrated" map of each park. The new Falcon park specific book is also by Cunningham. Combining the map and book led us to every place we wanted to explore.
The campground on the south side of Joshua Tree was the least busy during our fall visit a few years ago and did not fill during the week. We found a site at every campground we visited in the early afternoon during the week but many campgrounds did fill during the afternoon. Many rental RV's used Joshua Tree as the first stop heading north.