Kagami no Mukou no Saihate Toshokan

By the time he realized it, Wallace, the head librarian of the "Library of the End," had no memory. And what lies inside the forgotten library in the furthest end of the world are only a taciturn maid and the "Magic Monsters of the Books" that he just can't get through to. One day, he met Lucia, a witch apprentice from the village of the beginning, through a mirror. In order to save the world from the slowly encroaching demon lord. Lucia travels together with the hero who appeared in the village of the beginning. As Wallace is supporting Lucia from the shadow, he comes to realize that his past memory is the key to defeat the demon lord. This is a story of them, a lonely and gentle fantasy story.

(Source: Kadokawa, translated)