In the Library main catalog, use the term Letters as a subject term in one search box, and your topic as a keyword in the second search box.
Then limit the search results to your time period of interest. Limit to Books and Ebooks.
This same approach will work in WorldCat too, though I suggest searching Letters as a subject term only.
NOTE some of these are unpublished letters too, so pay attention to the source.
In the search results, limit to "letters" as a source type.