This is a rather fuzzily reasoned book, more so than this inherently fuzzy subject demands. His discovery of the Tabula Cebetis frontispiece was quite helpful for seeing the role of the Magician card. Chapter One, on Horapollo was eye-opening for me, as I had not realized how many correspondences to the cards there are. I also found Chapter 3, on numbers, worthwhile in that I had not examined some of his sources, such as Martianus Capella. The same is true in a lesser way for the chapter "Secret Symmetries". The second half of the book largely recapitulates what is in Wicked Pack of Cards, except when he tries to justify the assignment of meanings to the number cards in terms of the sefirot as characterized in a Jewish source, a venture that I don't think succeeded. But at least he tried, unlike most who have advanced the same idea. My personal working hypothesis, which seems to fit, is that the sefirot govern the trumps (which Decker denies, in favor of Pythagorean number symbolism) and Pythagorean number symbolism governs the number cards (which Decker denies, in favor of the sefirot).
I suspect that the meanings of the courts are partly the Pythagorean 1, 2, 3, 4, all over again (Page through King) in each suit, partly natural associations to these particular ranks in society of the times, and partly associations to particular personnages who were identified with these cards. In the French tradition they were given names drawn from French history and classical history and mythology. But I have not carried through this hypothesis to see if it fits Etteilla's meanings.