If by special publication you mean the Black Mountain Cave Survey carried out for the BBNP by SWCC (and others?) members, then it is indeed a mine of information but I don't think it's for sale. Most of the hydrological information in it is available elsewhere anyway (eg back issues of Descent - sorry not to be able to give refs but I'm too far away from my copies).
One thing that interests me is that if you combine the hydrological info with the geological map, you see that the water flows east-west and that, initially, at the Herberts end, the dip is about 10 degrees to the southwest whereas at the Llwchwr (IIRC) it's more than twice that and to the southeast. Given the size of the resurgence, I always thought that this meant that there was a shedload of water barrelling up-dip to get out and the only way that kind of pressure could build up was through long sumps (ie don't bother digging at the Llwchwr end but further back east). This seemed to get some support from Mike Barnes' discoveries at Heaven's Door but since I'm no expert, I'd be interested to hear other thoughts.