I have extremely limited information- not even an image for a cover, but this popped up on Amazon this week. This seems like it will be a material collection book on the establishment/development artwork and designs for both Zero no Kiseki and Ao no Kiseki.
This is also showing up on the Dengeki Guidebooks site, as well.
What’s known is this:
Price: 3360 yen
Publisher: ASCII Media Works
Release Date: 4/27/2012
I will add this to the calendar.