It ultimately depends on what you want to do with it. The Book of Thoth is widely acclaimed as the book, but if you're looking for divinatory meanings, it won't help you much, as it is both unnecessarily complicated for the average reader and also has much that is superfluous for people looking to the Thoth to do readings with. What it excels at is discussion of the occult and philosophical makeup of the cards, their underlying relationships with other cards, and especially their relation to Thelema. If you're looking for personal development through study of the cards, there is no better book, but be warned it is a study that could span several years. Not only that, but to understand it at all, you'll have to do a bit of preliminary research in Kabbalah. When writing the book, Crowley had perhaps a little too much faith in his readers' knowledge, and it might actually turn you off the Thoth altogether.
Book T is a little better, but isn't Thoth specific, and even the updated version LRichard mentioned is limited, to a great extent. This doesn't mean it isn't good, it is excellent, but I wouldn't recommend it to a new user just getting their bearings.
What I can recommend is Lon Milo Duuette's Understanding Aleister Crowley's Thoth Tarot. It is a great introduction, it gets you through the door, and discusses much of the preliminary knowledge that is needed. Later on, you decide if you actually want to step through that door (I did, and never looked back!).
I also recommend and urge you don't buy any of the "keywords" books by Banzhaf, Mirror of the Soul by Ziegler, and anything by Angeles Arrien. Those books simply aren't worth the paper they're printed on.
