There are two artbooks, Angel Cage and Lost Angel. All of the full color images from both of them are floating around the internet on different sites as far as I can tell, but I have not seen any of the other stuff scanned from Japanese versions let alone scanlated. Both of these books have been published in English by Viz, and the information I have about the non-artwork stuff does come from this version.
Angel Cage -- In addition to the color images from Angel Sanctuary, it also contains color images from some of of Kaori Yuki's other work, some black and white title pages, 2 pages dealing with the Lineage of Light (Heaven), 2 pages dealing with the Lineage Darkness (Hell), 1 page explaining some of the terminology in Angel Sanctuary, 5 page interview with Kaori Yuki, information on where the color pages were previously published and a postscript by Kaori Yuki.
Lost Angel -- This is a separate book with more color images that are entirely for Angel Sanctuary. About 1/2 of this book is just information on characters, story, another title page collection, another interview with Kaori Yuki and another postscript.
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