This book surprised me. While it is generally an overview and doesn't go all that deeply into any of the topics presented, it is a good start.
It follows three main 'branches' (Gaelic myth, Christian monasticism and Near Eastern thought - alchemy, astrology, etc.) that have influenced the stories about the Grail.
For example, W. B. Yeats observed a parallel between the Four Treasures of the Tuatha de Danaan and the four Aces of the Rider Waite deck. It continues:
"...And when we compare the four isolated aces from the Waite tarot pack with the four hallows of the Grail we do find that they have an uncanny resemblance to each other." The hallows and corresponding cards are Sword (Sword,) Paten (Pentacle,) Grail (Cup) and Spear (Wand.)
The book also tells how the best known version of the Grail story was probably influenced by the 'courts of love' in twelfth century France.
A good overview of the Grail stories. While there were a few dry spots, it certainly did it's job and sparked my interest to dig a little deeper into a few new areas as well.