How can one succinctly reply to this question?
Volume 1, which came out in 1978, encompasses a little of everything. As with the other two volumes (which increase in size), numerous decks are illustrated (mostly in black & white); pp387.
Volume 2 is quite an important resource in terms of Tarot's early history - and it also, of course, continues its vast coverage of Tarot decks; pp552.
Volume 3, whilst again covering a wide area, has a wonderful largish essay on Pamela Colman-Smith (the illustrator of the Waite deck); pp694.
Personally, and though both their indices and their bibliography are extremely useful, I find the way they are organised somewhat baffling, and lacking a structure which makes for easy cross-reference - though wonderful for perusal. This criticism is made, however, without being able to suggest how they could have been better organised!
Each volume is well worth getting.