Board Thread:Questions and Answers/@comment-173.185.123.84-20141129045446/@comment-25599611-20141129052725

the light novels only have 5 volumes released, so the cover art and illustration only available up to volume 5 for now. the rest of the volumes are released in form of web novel, which can be accessed here (it's in japanese).

also there're some additional stories in light novel witch aren't translated to english.

for more information, you can read it here