At the first thought I'd run out with Place, 'The Tarot: History, Symbolism and Divination'. But then I'd probably go back into the burning house to grab Rachel Pollack 'Forest of Souls' and also by Pollack '78 Degrees of Wisdom' I love all three almost equally. Just my opinion though.
Impossible to pick just one, but if i pick one to suit my mood just this week it will have to be "Tarot tells the Tale" by James Ricklef cause it is so delightfully fun and makes me wanna dive into tarot again after a long break.
As for companion books - few beat the book for the Tarot of Spirits by Eakins.
It would have to be Mary Greer's "21 Ways to Read a Tarot Card". So many excellent books have been mentioned here, but, for me, this one takes the prize despite very stiff competition.