I guess I'm old school. I don't own any other book about the Thoth Tarot besides the Book of Thoth. The only other book I ever bought on Thoth was Gerd Ziegler's book, in 1986 - I looked at it once, and I haven't seen it since.
If something in The Book of Thoth doesn't make sense, go to Crowley's sources, not to later interpreters. For the most part, that includes basic Golden Dawn doctrine and rituals, as well as Crowley's own writings. Almost everything you need to understand what Crowley means is actually available online right now. It was a lot harder in the 1980s, I'm sure you can imagine. But maybe that difficulty of access, finding sources, was a blessing in disguise. It forced one to actually read, and read, and reread, and digest, the writings. Chew that meat, chew through the gristle and gnaw on the occasional bone fragment, suck out the marrow. That's what being alone with the Book of Thoth forces you to do.
If you don't understand it, figure it out. Take your time. It's worth it. You'll learn a lot in the process of trying to figure out how to figure it out.
Everybody wants pabulum today, somebody who has simplified things for them. They want it quick, they want it NOW. But part of the point of learning the slow way is that the learning process itself is informative - YOU change as YOU figure out for YOURSELF what it means. You have to dig, you have to search, you have to think, you have to actually WORK to "get it."
My advice is to put aside every other book, and read Crowley and Crowley's sources, that's all. And, as he says, use the cards themselves. He wasn't using the Thoth Tarot, of course. This deck is what was in his mind when he was using a TdM or Wirth. Proper study of Tarot in the old occultist way, the way Crowley learned it, will give you one in your mind too.