The most popular area of the park is Hidden Valley/Real Hidden Valley. One of the most famous rocks is Intersection Rock. It is right off the park road you can't miss it.
Randy Vogel: Rock Climbing Joshua Tree (Falcon Guide) is the old bible for the park.
Robert Miramontes Joshua Tree Rock Climbs The new best book. This book may be out of print. But if you go to Nomad Ventures in the town Joshua Tree they may still have it. They will also have any climbing gear you need.
Mountain Project list of routes by difficulty
You should also bop on over to /r/Climbing and ask them for the best spots.