I would say that it is what it says: an introduction to the study of the tarot (as he sees it, of course). It is not a distillation of his ideas, rather, it is enough to get you going, so you can make your own discoveries and you don't have to read the chapters of "Imagiers" in any kind of order, but can consult it as a reference and way of going deeper.
A large part of the book, or booklet, is a rewrite of chapter 2 of "Imagiers", which, frankly, was a hard nut to crack, and extremely dry and formalistic. "Introduction" goes over the same material, but using black and white pictures of the cards, reproduced not once but twice, in different configurations with other cards, instead of the abstract diagrams of "Imagiers", and uses the relationships to bring out various aspects of the cards' meaning. He leaves out some of the relationships of "Imagiers", but it doesn't matter. Then he goes over the 22 again in consecutive order, just once instead of 3 or 4 times as in the later chapters of "Imagiers". You really do get an introduction to the whole 22. But if you want a separate account of each card in detail, you have to go to "Imagiers".
After that he restates some of his ideas about divination with the cards in what I think is a more practical and down to earth way than in "Imagiers". It seems to me that there is less emphasis on magic here than in "Imagiers"; in fact, I couldn't find it mentioned. There are cautions "not to claim the ability to pick up every vibration", and "quack promises are unworthy of the serious diviner, who should keep his statements within reasonable bounds" (p. 51). He warns against being too precise; it is rather that the reading should be such that "the client can then help to sort out his impressions and find the road to a satisfactory solution", perhaps thereby avoiding a mistake he was on the point of making. And if nothing else, "The sensible client will accept what is said with caution and wait until the import of the reading suddenly becomes clear to him." This last inspires the translator to give a long footnote about the self-deceptions of those who interpreted in their favor the vague oracles of Delphi and later realized their errors. A more familiar example would be the sudden realizations of Macbeth about the true meanings of the witches' predictions. I am not sure this is what Wirth had in mind.
So it's a good book to have, especially at $3 plus postage for a book long out of print and not in libraries. I ordered the original French, to see if the translation is accurate (it, too, was cheap, well, 30 Euros, not bad for a book that in the one U.S. library listing it is confined to the "rare book" room). I'll let people know. I didn't notice anything odd in the English, like I did with "Imagiers".