A number of the books in the Agro Gallery originated in the USA, though as far as the scans go we've usually opted for the UK editions. Justin, who is far better placed to comment on this subject, will tell you the whole UK Hells Angel pulp phenomenon was booted into life when Peter Haining reprinted Jan Hudson's 'non-fiction' account of The Sex & Savagery Of The Hells Angels (Greenleaf Classics, USA, 1966) as a NEL the following year; it went through at least ten reprints over here. Prior to the Angels, Skinheads, Suedeheads and Knuckle Girls, Juvenile Delinquent pulp was a thriving genre in itself on both sides of the pond.
From the first, I set myself against "literature"; the story was the thing, and no amount of style could persuade me to select a story that lacked genuine, unadulterated horror. For those who wanted something high-brow there was plenty.
Well I do have one "Juvie" collection of short stories by Harlan Ellison. I tend to think in terms of 1970's when it comes to paperbacks and I just don't recall any of those books around. Perhaps if I went further back, I would. It still amazes me how popular they were in the UK. Perhaps, like westerns, they're a genre which went from Ubiquitous to Extintion.