Associate Professor Akira Takatsuki's Conjecture, Vol. 3 (light novel)

The difference between damnation and celebration is paper-thin. At the same time, Takatsuki and Naoya learn of the urban legend about "Library Marie". Apparently if you solve a hidden cipher in the library collection, you'll be cursed. "The unlucky letter and cursed cipher." Upon investigating the legend of a demon god in a village, in a grotto they find a skull with a hole in it. Soon after, Takatsuki is plagued by unexpected suffering. How will Naoya help him out of this one?!