Japan: this is the title of a book of 162 pages, A5 sized (15x20,8 cm) released on the 9th of January by Glide Media: "the divinatory reference book of guardian angels" 守護天使占い図鑑 (ISBN 9784813022022 - excuse my poor English translation!). Written by the esoteric well-known author Moira Yuuki (read mo-i-ra), this book contains 98 illustrations made by talented artists: Kaori Yuki, Kai (also known as Nheira from the duo Pink Psycho, mangakas of the yaoi In the End) and Ichtys (mangaka of the fantasy shounen Superior). The cover is from Kaori Yuki and she also portrayed other angels inside the book. Now we understand why she had a 2-months break in automn during the writing of Iiki no Ki while she usually takes only 1 (to think I was hoping for an artbook, "ha ha").
EDIT Feb 9: I received a copy and sensei only made the cover. The picture is duplicated inside without text. A 4th illustrator/mangaka also worked on the projet: Nato Kio.

The book depicts the various known archangels and angels, tells how to know who is your guardian angel and how to recognize their special traits through yours, it also talks about demons and the deadly sins.