I recommend you purchase Tom Roster's Buffered Lead & Bismuth Shotshell Reloading Manual (3rd Ed).

You will learn more about the specifics of loading specialized and buffered shotshell loads, component substitution, buckshot for shot substitution, how to make buffer work and much more.  Indeed the only thing I disagree with Tom Roster on is roll crimping - he contends the top wad disrupts patterns, something I and a lot of other knowledgable shotgunners disagree with.  Also roll crimping gives more room for conventional and tri-ball type buckshot.

You may also want to check out the Casting for Shotguns section of:  http://castboolits.gunloads.com
Unlike many sites, much of the specialty data on shotgun loading is pressure tested and sources for having your loads pressure tested are noted as well.