Hey Lisa,
If you've never used the Thoth at all, I'd recommend Lon Milo Duquette's Understanding Aleister Crowley's Thoth Tarot to start. It will give you a lot of useful basics so that you understand the undderlying system a little more coherently. If you've been reading primarily with Waite-Smith base decks, be ready to toss out a lot of what you thoguht was "standard" in interpretation. Thought the WS and the Thoth are both based on Golden Dawn's Book T, they are literally worlds apart.
Eventually (and Duquette says this constantly in his own book), if you want to learn the Thoth properly, you must read Crowley's own companion text, but this is a great option for the beginner. If you're just starting with the Thoth, I strongly, strongly urge you to consider getting down to business with the Thoth on its own, card by card, for a while. And using the Duquette book alongside Crowley's Book of Thoth is well worth the time invested. Secondarily I'd recommend Snuffin's Thoth Companion which is complimentary to Duquette's book but less overlapping that you might imagine. Each of them provide a pragmatic path through the Thelemic woods. At the very least, these two books will help you to read Crowley's own dense text without swallowing your tongue.
If you do use the (good) Banzhaf books Psychebleu mentions above, you should know that Banzhaf is solid, but tends to be a little keyword-y and doesn't actually explain much of the deck in a way that helps you understand it; he pretty much intereprets cards and expects you to accept his reading whole whether you understand the logic or no.
Scion