This book has two parts, and therefore, uses.
The first part is that it is a guidebook used to interpret the deck of the same name, along with providing dire warnings about it's use. the inclusion of the artwork found on the cards is a nice addition, but for the full experience, one should buy the cards as well. (I disclaim any and all liability for using the deck for divination- if the book's warnings don't stop you, It's unlikely that I will be able to either.)
The second part is the story behind the redicovery of the 'Lost' Arcana cards, and the fate of the Archivist that was tasked with finding and recovering them. This story is a brooding, horror tale as told in letters, and it is equal to or better than that of Lovecraft.
If you are a fan of horror in the lovecraftian vein, Strange Oddities and Curiousities, then you should purchase this book (and the deck.)