Personally I don't really like Ziegler, and didn't find his book to be of much use. I have his book "Mirror of your relationships" (found it in a bargain bin) which is even worse than the other one.
The going recommendation is Lon Milo DuQuette's Understanding Aleister Crowley's Thoth Tarot which is a wonderful introduction. Be warned though, that if you're looking for "secrets," it is an introduction only, and you will (hopefully) reach a stage where it will be limited. However, as an introduction, to get you in the door, there is no better book. It towers high above any other introductory texts.
You might also want to check out DuQuette's The Chicken Qabalah of Rabbi Lamed Ben Clifford. Qabalah is the backbone of the Thoth (all Golden Dawn decks, for that matter) and in my opinion this book is just as important as his Thoth book. It will noticeably open the Thoth to you dramatically, really. It is a very friendly and personable book, explaining the basics of Qabalah in a completely un-threatening and approachable way. DuQuette is very, very good at making the obtuse digestible.
Apart from those, the real secrets are in Aleister Crowley's own Book of Thoth. It is free to download. However, it is a difficult book, and I couldn't make heads or tails of it before studying the two books I recommended above. Get those two, download the Book of Thoth and you will have practically all you need for many years of happy study.