This is the official artbook released with the limited edition of the 29th volume of the official manga. It contains a lot of fan art from a variety of artists.

My apologies for the scan quality, but this was the best I'd seen so far.

There is also a second artbook, "Girls Graphics", that was also a limited edition release, but that one has not been scanned yet to my knowledge. If someone finds decent scans and uploads them, I'd be grateful. It has official promotional artwork that can mostly be found on pages like danbooru already however.

For some reason, file 21 does not seem to get a proper thumbnail, I'm afraid I have no idea why.