Well, this is getting away from the Planets a bit, but maybe Similia will shift us into another new thread.
The astrology of the Golden Dawn is focused on horary and natal... and as such the founders would have been looking at most of the biggies, which in turn would have led to the writings of more obscure astrologers: the
Tetrabiblos, the
Corpus Hermetica, Dorotheus, Manilius, the Neoplatonists, the Stoics, the
Picatrix (most likely in Lilly's Latin version)... which would likely have been the source for the Arabs (Al Biruni, Ibn Ezra, Mash'allah, Albumasar, etc... but I'd have to check because the GD may have had direct access), Guido Bonatti, William Lilly (his amazing
Christian Astrology is available all over the net as a pdf)...
ad infinitum The thing is, up through the 17th century, astrologers actually studied and knew their sources. So it's a fabric: tug at a thread and you'll find they're all connected. But Lilly would be a great place to start because he's a wonderful writer who loved combining study wth practice... and in many ways served as the headstone on the tradition as it was virtually slaughtered by scientific materialism.
Interestingly, though much of the GD teachings come straight from Agrippa (by way of Barrett's plagiarized material in
The Magus), they don't take it wholesale. As Kwaw pointed out to me a while back, the GD decan system's planetary order is the one identified in the
Picatrix as the Chaldean system, which deviates consdierably from the "Vedic" rulerships presented by Agrippa.
Also worth noting is that the astrology taught by the GD expanded between the 1st and 2nd orders (shifting from tropical to sidereal); this shift turns up in the use of Cor Leonis as a "starting point" for certain attributions.
A couple pages to check out:
http://www.societyofastrologers.com/whatistraditionalastrology.html
http://www.renaissanceastrology.com/traditionalastrology.html
http://www.new-library.com/zoller/
http://www.bendykes.com/
http://www.projecthindsight.com/index.html
http://www.leelehman.com/
http://www.skyscript.co.uk/
http://carnesoft.quotaless.com/ebooks.html (Free e-books)
http://www.worldastrology.net/
http://www.classicalastrologer.com/
A wonderful overview/interview with Ben Dykes at
http://www.skyscript.co.uk/bdykes.html