Some of you may be familiar with Johnny Cash's 1968 Folsom Prison album. At that concert was a prisoner named Glen Sherley. During the concert, Johnny Cash famously played a song of Sherley's - "Greystone Chapel" - that he'd heard a tape of the night before. Sherley got out of prison in 1971, and with Cash's help attained some success. But in the end those demons got him, and he committed suicide in 1978.
At any rate, Sherley was the real deal. Cash sang a lot about prison, true, but Sherley had spent over a decade behind bars. Next to Sherley, Merle Haggard comes closest, having spent about 2 years locked up.
Anyway, I'm convinced that Sherley was as talented, both as a songwriter and performer, as Haggard, Nelson, and Cash. But he managed to produce only one full album before the demons got him. This one, recorded live in 1971:
At the very least, listen to this song from the album:
[url=http://www.youtube.com/watch?v=EzkH4dpeAdQ]If This Prison Yard Could Talk
I recommend then listening to the whole excellent album via this playlist: Whole album