What is the blue box that holds the book

The blue box in which a book is mounted is often referred to as a **correspondence sleeve**.

The letter jacket is a traditional book protector, made of thick board paper for the lining and a box jacket framed with fabrics such as cloth or brocade, into which the book is mounted, with a toothpick or bamboo skewer as the encapsulating tether. The main purpose of using the blue box for books is to provide further protection measures for books, this box is also made specifically for each book.