The official subject heading for letters is "correspondence" though sometimes it is listed as correspondences. I suggest using correspondence* as a subject. Combined this with an individual or topic. Do NOT limit the publication date; most collections of letters are compiled publications print well after the fact, so you'll miss important materials.
You can also search for a name or topic, and search Letters within the Title field. This isn't as exact as the above, but still possibly useful.
Letters
Letters is not the correct subject according to the Library of Congress, but this is used somethings by catalogers. If you're really stuck, try Letters as a subject. Interesting, "love letters" is, in fact, a correct subject heading -- even catalogers must have realized "love correspondences" sounded silly.
The majority of our databases for letters specialize in unpublished materials
Highlights