The books that helped me were Pete Seeger's HOW TO PLAY THE FIVE-STRING BANJO and Eric Mueller's FRAILING: AN INSTRUCTIONAL MANUAL (a Mel Bay Product.) Brad Leftwich has an excellent instruction book called ROUND PEAK STYLE BANJO or something like that: it includes a CD recording of each exercise, for those like me who cannot learn without hearing a demonstration. Seeger's book only devotes a few pages to clawhammer, but his explanation is good. If you can find a festival with an old-time banjo workshop, that would be best of all. There are also instructional videos from Ralph Stanley, Gary Brewer and others, but I don't know enough to recommend them.