Personally, I do not think he is a
great writer - but remains a very
important author - on Tarot. Two of his earlier books (the first on the 1jj, the other called
Tarot Classic) are also worth getting, but they assume much and make errors.
Where Kaplan is indispensible is with the Encyclopedias. Essentially, he has put at our fingertips such a wealth of information that he has provided an invaluable service to the Tarot Community (whilst also making a few millions through his US-Games)...
His 'habit' of requiring that artists hand over the originals for decks, by the way, is certainly not quite correct, for the creator of the Spiral deck still has, to my knowledge, her originals.
I do not know what the state of his collection is now, having apparently sold a number of materials from his personal collection, but it would still be one of
the best Tarot collections in the world.
...but back to the books - I would definitely recommend his Encyclopedias - including the fourth volume, sight unseen, due soonish