This is a Japanese-language illustration (art) book.
None of them are ever completely naked, but the "seishoujo" ("saint" girls) of the game of the same name do have an awful lot of sex. Which is of course the point of the game. Very nicely done illustrations are accompanied by sketches, conte, and the text that appeared in that scene in the game. Naturally all the naughty bits are censored, but this book is definitely not for the underaged. 72 pages; hard cover.