My interest in alchemy lies mainly in better understanding the GD system. In the books I have read things like Kabbalah and astrology are dealt with exhaustively, but alchemy is either treated as if everyone already knows it or, of course, mentioned in passing as unique to this or that grade. I have been eying the Holy Light as a good demonstration of alchemical concepts. All I really know of alchemy is how the basic three elements in the GD are pushed everywhere.
Adam's alchemy site looks like a good resource.
I don't have much in the way of alchemical writing either. Buried in Israel Regardie's massive Falcon Press tome
The Complete Golden Dawn System of Magic, at the very end of Volume Two, are two articles on alchemy, one by Regardie and another by his friend, Hans W. Nintzel. The latter cites numerous other writers on alchemy, some older - Paracelsus, Geber, Glauber, Vaughn, Sendivogius, Flammel, Isaac of Holland - and others more recent - Frater Albertus, Phillip Hurley and Archibald Cockren.
In his nearly as massive
The Secret Teachings of All Ages, Manly P. Hall devotes 58 pages to alchemy, including two sections titled Theory and Practice I and II. In addition to many of the above luminaries, Hall mentions Raymond Lully, Thomas Norton, Basil Valentine, Jean de Meung, Roger Bacon, Picus de Mirandola, John Dee, John Frederick Helvetius, Alexander Sethon, Count Bernard of Treviso, Sir George Ripley, the Comte di Cagliostro, the Comte de St.-Germain and a host of others too numerous to list. Quite a few of these names I recognize, but I haven't (knowingly) read any of their work.
There is quite a bit on alchemy scattered about Agrippa's
Three Books of Occult Philosophy but it's kind of a pain to dredge it out using the index.
I also have Paul Foster Case's
The True and Invisible Rosicrucian Order. Although it has many references to alchemy throughout, it's primarily about the Rosicrucian Allegory and the Grades of the Order. I understand Case was a latter-day GD member, so this one might be of some use to you.