'unknown author' is correct and can be used in written English.
For example, one might say, "This quotation is attributed to an unknown author."
125 140 by an unknown author.
Her famous book club could turn an unknown author into a bestseller overnight.
Not bad for an ostensibly lightweight novel by an unknown author.
The style of the author of "Author Unknown" and the letter's unknown author match up nicely.
For a previously unpublished, completely unknown author to sell two million copies — that got everyone's attention".
How would one go about valuing an unpublished masterpiece by an unknown author?
After two years, the penny was beginning to drop that normal publication for an unknown author is virtually impossible.
As the work of a relatively unknown author, neither the Observer nor the Manchester Guardian saw fit to review it.
Michinaga also inspired still another contemporary romance, the Eiga monogatari (A Tale of Flowering Fortunes, 1980), by an unknown author.
He had not yet completed that work when a bogus sequel by an unknown author was published in 1614.
The superintendent predicted that the unknown author would fail to show up, and that "the staff here at Five Points will all know what a punk you are".
