The library sure is dark even though there are a lot of lights. It still gives a nice feel to the image and there are so many details to look at.

The perspective on Koakuma bothers me a little. She's reaching for a book on the farthest column of the bookshelf, while Remilia is sitting on the on the furthest side of the column next to it and with that Implied Pantyshot-tag, Koakuma would have to be leaning pretty far back, in reference to her orientation to the bookshelf.
Some perspective foreshortening would have worked wonders on her lower half.