It's a 192-page paperback filled with illustrations from all four Disgaea games, namely Disgaea: Hour of Darkness, Disgaea 2: Cursed Memories, Disgaea 3: Absence of Justice and Disgaea 4: A Promise Unforgotten.
There are illustrations from all sorts of merchandise like game covers, books, soundtrack, other promotional materials and in-game illustrations as well. It's not a concept art book.
The art is beautiful, printed in mixture of sizes from quarter page to full page. It's shonen manga style with very pastel colours throughout. The bulk of art comes from Takehito Harada. However, there are many illustrations drawn by other artists who were not credited. The index page have info on where the art came from. The artist commentary appear for only three illustrations.
This book is translated by Udon from the original Japanese edition (isbn 4048706373) that was published in 2012. That book had mixed reviews, citing the lack of artist credits and duplication of work from other books such as Takehito Harada Art works (isbn 475806265X) and Disgaea 3 Official Design Works (isbn 4797351519). If you don't have those books, then it's not a concern.
If you like the Disgaea series and the style of art, this is a great buy.
(There are more pictures of the book on my blog. Just visit my Amazon profile for the link.)