Zeikin de Katta Hon

Synopsis (en anglais)

Ishidaira is a delinquent who visited the library for the first time since elementary school. But then, he was pointed out by Hayasameru and Shirai, who works at the library, that he has not returned a book that he borrowed ten years ago. A manga about how Ishidaira went from trying to borrow a book from a library to working there instead. (Source: Kodansha, translated)