Associate Professor Akira Takatsuki's Conjecture Vol 5

From the Yen Press edition

After closing the case on the eerie mystery of 4:44, the professor and his intrepid intern investigate reports of mermaid sightings in a seaside town. As Professor Akira would be quick to explain, not all mermaids are gentle beauties— and as Fukamachi can prove, not all mysteries are built of lies… And their adventures don’t end there, as a lead from the past soon sends them tumbling into the candlelit terror of the Night of One Hundred Horrors!

Source: Yen Press