Richard
I have Pollack's 78 Degrees. It's better than most other tarot books and certainly worth reading, but I don't think any single book is the ultimate authority. The ultimate authority is YOU!
The 78 Degrees of Wisdom is a must have. Now and then i always take a look at it and got something i never saw before.
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.