78 Degrees of Wisdom was one of my first tarot books; it a very thorough book on the RWS deck, with for some cards more attention to the little details than even an ant's accountant could wish for....
It's a classic, but I feel it is a bit on the dry side sometimes and some of her interpretations felt a bit far-fetched to me.
(I am liking Paul Quinn's 'Tarot for Life' much better as a 'standard' book on the RWS deck; I really like the way he writes, his takes on the cards make sense to me and the book reads wonderfully, especially the little anecdotes for each card where he describes it coming up in a reading. But, of course, Paul Quinn is not Rachel Pollack, and you asked which Rachel Pollack book to buy!
)
I like Tarot Wisdom a bit better I think; it is broader in scope (although I also miss the Thoth, it's like a very quiet invisible elephant in the room!) and has some interesting spreads.
But by far my favorite of her books is 'The Forest of Souls'.
Someone upthread said that Ms Pollack is more like a philosopher on tarot, which I agree with; and this book is her philosophical masterpiece, I think.
It is not about card meanings, but instead treats tarot like a spiritual tool, a way to ask and anwer questions about... Life,the Universe and Everything, really.
It is very interesting and thought-provoking and it's one of the few (tarot) books which I have re-read once or twice over the years, and I will probably re-read it again in the near future.
Of course, if this thread goes on long enough, eventually every one of her books will be named as a favorite by someone!
Edit: 78 Degrees is a book that's very popular, and I've seen it turn up second hand in loads of places - you should be able to pick it up quite cheaply somewhere. If there aren't any brick & mortar second hand bookstores near you, betterworldbooks online is one of my favorites.