This book is an excellent companion to the Thoth Tarot Deck - if you are new to the Tarot or if you want to do divination with the Thoth deck. Crowley's "Book of Thoth" is a rather meaty tome, and can be daunting to the beginner. To add to this, "Book of Thoth" focuses on the information useful for meditation, pathworking and Magickal theory.
"Mirror" is far more practical in its approach. Each card is reproduced (unfortunately in Black & White rather than Color) with a set of keywords, a basic description of the symbology in the artwork as it applies to a comprehension of the card for divination, and a set of rather new-agey affirmations and excercises (this is my only complaint with the book, however, they make it more accessable, they are small, and I will admit to having used them on occasion). The book ends with some fun items, such as how to find your soul card, & the obligatory layouts.
I have not been exposed to any of his other work, but I just noticed that he has written several other books on the Thoth Deck. This reinforces my initial opinion that "Mirrors" was written to share and make accessable the Thoth Deck and not just to make a buck by invoking Crowley's name.
I recommend getting this book and once you are moderately familiar, grab the "Book of Thoth" to fill in the true depths of most likely the greatest Tarot Deck of all time.