The growth rings show the age of a tree, representing the history. However, It's not practical to use real wood to make a book cover or case, Instead, material looking like wood was used for the book cover, with the official logo 100th anniversary imprinted on it to accentuate the effect. For the book cases, blue is used as it's a color which is frequently used in formal occasions, and it's also main color of Nihon University College of Art Logo.
