Any time you need a dynamic dropdown view filter, you need a Taxonomy vocabulary (Drupal core).
In your example, you need first to create a vocabulary named Authors (no need to enter terms).
Secondly, you add a field Author, type Term reference, related to your vocabulary Authors, in the content type displayed in your View.
Choose autocomplete widget to allow term creation when a content is created.
Now, if you create a new content and enter a value in the field Author, it will add each new term or suggest existing ones.
Finally, in your View, you add an exposed filter related to your field Author, and you can select dropdown list. All authors entered during content creation will appear in the list.
If you need to enter manually authors, you just have to get to your Vocabulary Authors and add terms. They will also appear in your dropdown filter.
Hope this helps.