I finally tracked down a copy of the 1936 edition of Gundel's
Dekane und Dekansternbilder at the Columbia University libraries!!!! (thank god for alum privileges) ... which is essentially the ONLY source devoted to the history and theories about the Decans and their use. A colossal amount of information, and something that postdates the Golden Dawn, so much collation that would have been useful/powerful back in the day...
Sorry for the delay on printing and sending version 2.3 of the Guide... I've been stalling as I try to translate some murky Gundel chunks. It's in German so it's going to be slow going for me, but there's gold in them thar hills! And the Ars Memoria/Metrodorus/Bruno stuff is busting my head open.
Scion