One or more books are the focus (the subject, the theme) of the image. Use this tag when books are very conspicuous, very noticeable, very visible.
Examples
As guidelines, use this tag when a great portion of the image is occupied by:
A whole book.
A page of a book.
Many books.
Someone reading — or writing, or showing, or reacting to, or hiding behind, or otherwise paying attention to — a very visible book, that is preferably close to the center of the image.
