The xxxHOLiC Post Card (two words, really?) Book is literally a book made up of postcards. All 19 manga volume cover illustrations are printed on high quality, thick, smooth, postcard-like paper. The illustrations are unobstructed by text, so you can enjoy CLAMP's beautiful illustrations of Yuuko & Watanuki. The size of the book is equal to that of the size of the xxxHOLiC manga tankouban. The postcard pages are bound together in a way that the cards are easy to pull out, so that you can mail these to a friend or frame them. The series had some of the best artwork in all of CLAMP's repertoire, and CLAMP definitely had fun dressing Watanuki & Yuuko up in different styles of clothing. These would look very lovely in frames.
I would not normally pay $35 USD for 19 small-sized pages, but hey, I was at Kinokuniya in NYC, and in shopping mode. I'm fairly certain that this is supposed to hold fans over until CLAMP releases an artbook for xxxHOLiC (which will come out this winter). Until then, this will do.