coauthor; joint author; collaborator
Refers to a person who jointly creates a written work, such as a book, article, or report.
This book was written by three coauthors.
He is listed as a coauthor of the paper.
Very similar in meaning, but 共著者 is more common in bibliographic contexts.
Compound of 共 (きょう, 'together') + 作者 (さくしゃ, 'author').