"The Unknown," by Riley Sager.

The literary work titled The Unknown has been authored by Riley Sager. This specific publication is released through the Penguin Publishing Group and TNS. The sources confirm the authorship of Riley Sager for this title, ensuring that the book is associated with both the mentioned publisher and the distribution entity. No further plot details or release dates were provided in the sources, focusing solely on the identification of the author and the publishing houses involved.