If a library has more than one nonfiction work on the same subject, the

books are filed
a. by call number first, then alphabetically by author's name.
b. by call number first, then alphabetically by title.

a. by call number first, then alphabetically by author's name.

The books in a library that have more than one nonfiction work on the same subject are typically filed by call number first, and then alphabetically by the author's name. Therefore, option a. "by call number first, then alphabetically by author's name" is the correct answer.