I don't think a book was ever written about Carno Adit and it's associated cave system. If there was and anyone knows what it was called and the author, I would be interested to know so that I can see if I can find a copy.alastairgott said:
The other alternative is the Pwll Du tunnel. This was opened back up a few years ago but sealed again as it is collapsed and dangerous although not sure a book was ever published about it. The only other one I can think of is the Waun Felen and Tranch iron mine that was explored and surveyed by Brynmawr Caving Club members when it was broken into by roadworks and a book about it published by Bill Gascoine. Access is still possible but finding a key can be difficult and bad air at the end makes it dangerous. It also isn't very close to Llangattock.
I can't off the top of my head think of any others.