I think it's online but as a Devilman fan I recommend reading Neo Devilman. Neo Devilman is an anthology which is full of short stories revolving around or taking place within the Devilman story. And all of the authors are well known mangaka. In fact Go Nagai has a story in it, which will seem very familiar because it was so well recieved that he included it in the original Devilman story and subsequently all volumes were reprinted with the short story becoming a part of the larger narrative.
Ah, also if you can find it Amon: The Darkside of the Devilman is really fun too and has great art. It's a two part story, the first part takes place before the final battle and is about Amon (the demon who is possessed by Devilman) taking control and the second part is all about his origins in prehistory.
Holy shit, I didn't know about this but apparently Amon had a 'sequel' same artist and all called Devilman Mokushiroku (Apocalypse) - Strange Days. And hey whaddya know it's all nice and scanlated online. While I guess it's a sidestory, it rather seems like a reimagining in a way. It's a pretty self contained story within the Devilman setting.
99 Problems and a Cat
Croi Desai vs. HR99
@ 12:30 AM Apr 23rd