I have a page listing dozens and dozens of authors. These can be individually found from anywhere on the site using the header search box.
However I was wondering if, when users come across this particular page full of authors, it could be useful to include a second, internal search box just for this list. Is it a good thing from a UX perspective?
I believe there may be a case where this could be acceptable. It seems like you are thinking more of a filter specific to the authors content area, rather than a second search. You would need to work to distinguish the two components to ensure user expectations were clear, Filter vs. Search.
Assuming visitors to the authors page were looking for an individual author, an internal filter could work well. As the user types the content, the list of authors are filtered. Similar to the functionality in this example - http://codepen.io/JFarrow/pen/DwKap
If your site-wide search returns results as the user types (like Amazon) then maybe you are better working on categorising the real time results so user's can quickly & easily find authors from there.
A second search box would be confusing, especially if it were to filter the same result. The user would ask "why can't the first search box do the same thing as the second search box"
What you need is:
Pagination. Pages typically have "10", "20", "50", "100" or "All" options to adjust the page size. That way your used won't be scrolling through very many items. Works good for page rendering performance as well.
Filters. If the results can be subcategorized (like by author, or by year), then use filters to let the user pick out which ones to funnel in. That's a ton easier than typing in a second search box. In fact, it's a scroll and click away.
Sort. Never forget sort. The last thing you would want the user to do is scroll back and forth in a list that is not organized in any way.
Typical example: E-commerce websites like Amazon, E-bay, BestBuy etc.
