The World of Disgaea is a standard Japanese fan book. It contains a few promotional images and an assortment of character designs, animation sketches and backgrounds. The images were a bit too small for me (thumbnail size - 1/4 of the page). The page layouts was done well.
One-third of the book is dedicated in detailing facts that can be found in a game guide. It lists the status of different character classes and monsters, as well as equipment. There is also a summary of the plot and describes the multiple endings. All these things doesn't overpower the artwork though.
I find myself opening it once and a while, so I like it. Compared to the average costs of these sort of books, its a good deal.