The Book of Thoth is absolutely indispensable to the serious student of the Thoth Tarot. It is the ultimate source of ideas about the Thoth deck. Just buy it, buy it, buy it!!!
Now a word of warning. Most modern Tarot books are written for as wide an audience as possible. Essentially they have to reach down to the lowest rung on the ladder and be inclusive of idiots and morons.
Crowley doesn't do this. He treats you as an intelligent person right from the start, who is prepared to study and do a little research. Unfortunately this is rarely the case these days and you end up hearing some real gems like, "Crowley is a bad writer because I can't understand a word he says".
Several members of this forum, myself included, started off with nothing more than The Book of Thoth and we still survived. But these days it doesn't have to be such a rough ride. A kind soul called Lon Milo DuQuette has written a brilliant introductory book called, "
Understanding Aleister Crowley's Thoth Tarot", that gently eases the new reader into the wonderful world of The Book of Thoth.
Take the plunge!
P.S. On all initiatory journeys the hero is nearly always beset by demons tempting him/her to wander from the path of the wise. These demons are called Gerd Ziegler, Hajo Banzhaf, and Angeles Arrien. This evil triad will constantly try to seduce thee and muddle thy mind with New Age clap-trap, mental diarrhoea, and pure fantasy. Beware!