I've been keeping up with news of Steve Earle and lately, on Earle's main site, learned that the newest bio-documentary on Johnny Cash includes an interview with Earle about Cash. I'm DVR'ing the bio on Biography (cable) and will catch Earle talking influences--Earle who has been far more overtly political than Johnny C ever was but otherwise derives a great deal from this gone elder.
I also recommend this YouTube video clip - a contemporary montage of Cash's dark view.