>>98411
Manly P Hall is a good writer in his own right though his wider books such as STAA can be a little vague and mostly serve as just an introduction and reference to various beginnings of esotericism. If you want to get into the meat of his research, philosophy and practices then you should definitley check out his lectures which are still widely available for download and on youtube. There are 100s of them and some of them are really illuminating whilst not always being about the occult and sometimes just going into esoteric anthropology.
What you want to do is instead of trying to find a hard copy of the Gospel of Thomas (which will be tiny) is buy the complete Nag Hammadi scriptures in one volume. You can get them very cheap on amazon in some great editions which are annotated to provide full context and also contain introductory essays by the scholars and translators, maybe you don't plan to read them now, though they definitley provide a lot of context over that which you would have of just picking up the Gospel of Thomas and never heard of it - if you did that it would confuse the shit out of you. A very concise, but very well written and argumented, is Elain Pagels "The Gnostic Gospels." This book will go through the history of the early christian era, examining the arguments between the Orthodox and Gnostic sects, their political power and outcomes, and go into depth in some areas of the Nag Hammadi itself in a very easy to grasp and read format from any level of understanding on the subject. It is not all comprehensive but it is an excellent place to begin and get a good insight.
The final reccomendation that I have is a 4 part documentary sereis released in the UK in the 90's on TV called "Gnostics". You can find them all on youtube also and they are very well made, sometimes reenacting historical events and speeches, interviewing the leading scholars and historians on the subject but giving you a really great narrative and story with in each episode. It extends from stuff like the Cathars, to the Medicis, to Jung, you can watch them all here:
Gnostics 1/4 - Knowledge of the Heart - https://youtu.be/t6RLWbOSvUw
Gnostics 2/4 - Cathars, the True Christians - https://youtu.be/q8iuWixkFXw
Gnostics 3/4 - The Divinity of Man - https://youtu.be/0lGH785eI-E
Gnostics 4/4 - Crack in the Universe - https://youtu.be/JxxxmXkJums
After taking your time going through some of this work and a bit of contemplation I am sure all of your basic questions on Gnosticism will be answered, although be prepared to have many many more questions once you have them, the rabbit hole is particularly deep.