When writing a paper and reference book that have more than one author?

answers1:If the book has multiple authors and that's all you know,
then use authors (plural).If it has multiple authors but within that
you're told who the author of each chapter is, then use author
(singular).
answers2:If more than one author wrote chapter four, then yes,
"authors" should be plural. If your book is an anthology where one
author worked on each chapter, you should only refer to the author who
wrote chapter four. In that case, it would be singular. If the book
doesn't say who wrote what chapter, use the plural.