An infrequently-used age label indicating a book is intended for adult readers.
The label "adult" is not often used on books. When other age labels (such as Young Adult) are absent, a book is often assumed to be for an adult audience.
However, under certain circumstances the word "adult" may be used for emphasis or clarification.
First, if the title or premise of the book sound as if they are intended for a younger audience, the word "adult" can be used to clarify that the book is intended for adult readers (and possibly inappropriate for younger readers).
Second, if a book contains content or themes that are generally thought of as inappropriate for younger readers, such as explicit sexual content, graphic violence, or profanity, the word "adult" can give added emphasis that the book is not appropriate for younger readers. This can also be accomplished through a more specific content warning.