Anthony Louis, The Fool's Journey Around the Zodiac
I just finished reading it on my kindle. It's available for free in the kindle unlimited basket, but I decided to buy it because I found it useful.
This is a very nice and handy summary of the decan system, and I like that it doesn't pretend to do anything else. It's easy to use for a study of the Thoth, especially the minors.
I went one by one through the zodiac (which I have done many times before, I love playing with my Thoth and sorting it according to planets, colors, symbols etc) and had a good look at the cards associated directly or indirectly with one sign after the other, majors and minors and the court cards ruling the decan. I tried myself to find the common denominators and the individual traits of each card. Then I read it up in the book.
The book makes it easy because it also adds the tree of life influences, including the planets.
Altogether, this gives each card a very complex character - different energies are present, some are felt quite easily, others just appears as planetary or elementary echo. Even though I felt that I know the Thoth quite well, it was a good exercise to go through the book sign by sign and concentrate on that aspect.
I don't think one book is enough to understand the complex relationship and associations between kabbalah, astrology and tarot, and this book wouldn't be enough either. But while most other books go through the deck card by card, this one is organized sign by sign which makes the information easily accessible.
Louis doesn't write for a specific deck; I just found his approach suited the Thoth very well, but his interpretations would also help get an additional aspect in RWS studies, and I will do the same exercise with my Commemorative deck soon.
My copy is already annotated which is always a good sign. (My most annotated book is probably Benebell Wen's Holistic Tarot ;-) ) Over time, I'm sure I will add to these annotations.