Shiori no Kotoha ~Misshitsu Toshokan no Ikenie~

At the end of a deep forest, there was a large library with European-style architecture.
Azuma Kenji drops by the library whenever he finds time. The reason why Kenji, who doesn't usually read books, comes by so much is because of the girl who works at the library, Tokita Shiori.
Kenji falls in love with her at first sight, and spends his days in the library reading and watching Shiori. One day, Kenji falls asleep and is awakened by the sound of an emergency bell...
The area was pitch black and the doors would not open...the library had turned into an inescapable prison.
Kenji's life changes drastically as a result of his involvement with her...

[Translated from DLsite]