General terms:
* Anonymous book: This is the most common and straightforward term.
* Unattributed book: This emphasizes the lack of attribution to a specific author.
* Work of unknown authorship: This is a more formal way of saying it.
Specific terms:
* Pseudonymous book: This refers to a book written under a fake name.
* Folklore: This refers to traditional stories and beliefs passed down orally, often with no known author.
* Anonymous classic: This refers to a famous book with no known author, such as "Beowulf" or "The Canterbury Tales."
Other terms:
* Lost author: This is used when an author's identity is unknown but there are clues or evidence suggesting they may have existed.
* Mystery author: This is used when the author's identity is a mystery and a subject of speculation.
The best term to use depends on the specific context and the reason for the book's anonymity.