In the study of any science (and I consider Tarology to be one), nothing helps understanding more than surveying its history, most of all the milestones in its development. Besides using a good “hitchhiker’s guide” (such as Ronald Decker: A History of the Occult Tarot), I regard the following texts as the most important ones for tracing Golden Dawn based Tarot back to its roots.
The earliest outline of the GD system in general and its Tarot in particular.
The source for the attributions of zodiacal signs and Hebrew letters in Cipher MS.
The Book of Thoth