Angie Arrien was my breakthrough with the Thoth deck (not that it would be everyone's). Jim Wanless took the same class with her that I did, back in the late 70s in San Francisco. It was his first real intro to tarot. I was teaching the RWS deck at a local college. Anyway, I found that Angie helped me to finally be able to read with the deck - especially by encouraging an astrological approach. I went back (for about the 3rd or 4th time) to Crowley's Book of Thoth and noted how much Angie had changed the material. I struggled to integrate the two, and found that some of Angie's material has stayed with me as a helpful extra dimension to some of the cards, but that, by contrast, I could finally *get* Crowley. So, truly, Angie's "errors" opened up an appreciation and understanding of Crowley's book to me in a way nothing else had ever done.
Then I studied the deck Qabalistically with Diane di Prima, who was inspired by Mayananda's _The Tarot for Today_ (based closely on Crowley). She also got me into meditation on the Tattvas and Court Cards.
Also, I found that going back to the GD's Book T was very helpful, as was Crowley's astrological text in Evangeline Adam's _Astrology: Your Place Among the Stars_.
So, I'd say that the Thoth deck is a life-long journey with many surprising turns. It's ideal to approach it at different times in your life from totally new directions and to periodically take a break from it so that the material can be absorbed within. But, the most important thing is to find a way to actually read with it so that the cards come to speak to you directly. Use anything that will facilitate that. And always, after anything else you read, go back to Crowley's _Book of Thoth_.
I'd love to hear details of other people's journey in coming to understand this deck.
Edited to add: I really like duQuette's book, but he is giving you a highly concentrated, if beautifully organized, course in GD/Crowley teachings. Keep his book handy as it will make more sense eventually. My only complaint is that much more could have been done with the Minor Arcana.
Gerd Zigler's book is actually a well-and-highly-edited transcript of the course I took with Angie (several people were taping and transcribing the material). I have places from my notes in the class that match his text word-for-word, and it's Angie's very unique approach including stories I've only heard in these two places (and I've searched!).
Mary