I've been loading BP in cartridges since the early 1980's. May I recommend the "SPG Black Powder Cartridge Loading Primer". Also, anything by Paul Matthews is good.

If this is your first endeavor into double rifles, "Shooting the British Double Rifle" by Grahame Wright is the best I know of. It helped me immensely when I bought my first double.

NFB loads, J.C. Munnell in the now defunct Double Gun Journal had some. Most of their back issues are available but I don't know which issue they are in.

NRA Benefactor 2008
NRA Patron 2007
NRA Endowment 1996
NRA Life 1988