In this example, you could substitute "journal" or any other documents that could be considered to be private or personal. There are several serious issues concerning the status of these and many other such documents under the current United States copyright law. The first and most important consideration is whether or not the documents (called works by the Copyright Law) have been previously published. If you found the documents in essentially their original condition, it is unlikely that they would be considered to have been published previously. The term of copyright coverage and the ownership of the copyright are all governed by the U.S. Copyright Law. In addition, there may be real privacy concerns, especially if the letters contain information about people who are still living.
For an interesting discussion of some of these issues, see the following article:
Jeffrey L. Harrison, Privacy, Copyright, and Letters, 3 Elon L. Rev. 161 (2012), available at http://scholarship.law.ufl.edu/facultypub/352
You might also want to consult the following article:
Privacy issues are rather slippery. But even though relatively complex and subject to interpretation by the Federal Courts, copyright law is more clearly defined than any privacy issues. In this case, I will once again refer to the Cornell University chart entitled, "Copyright Term and the Public Domain in the United States, 1 January 2015." As noted by the Chart itself:
This chart was first published in Peter B. Hirtle, "Recent Changes To The Copyright Law: Copyright Term Extension," Archival Outlook, January/February 1999. This version is current as of 1 January 2015 . The most recent version is found at http://www.copyright.cornell.edu/resources/publicdomain.cfm. For some explanation on how to use the chart and complications hidden in it, see Peter B. Hirtle, "When is 1923 Going to Arrive and Other Complications of the U.S. Public Domain," Searcher (Sept 2012).This footnote from the Chart should give you a more than adequate start at resolving any questions about copyright law in the context of any genealogical concerns.
The chart is based in part on Laura N. Gasaway's chart, "When Works Pass Into the Public Domain," at <http://www.unc.edu/~unclng/public-d.htm>, and similar charts found in Marie C. Malaro, A Legal Primer On Managing Museum Collections (Washington, D.C.: Smithsonian Institution Press, 1998): 155-156. A useful copyright duration chart by Mary Minow, organized by year, is found at <http://www.librarylaw.com/DigitizationTable.htm>. A "flow chart" for copyright duration is found at <http://sunsteinlaw.com/practices/copyright-portfolio-development/copyright-pointers/copyright-flowchart/>, and a “tree-view” chart on copyright is at <http://chart.copyrightdata.com>. Several U.S. copyright duration calculators are available online, including the Public Domain Sherpa (http://www.publicdomainsherpa.com/calculator.html) and the Durationator (in beta at http://www.durationator.com/). Europeana’s public domain calculators for 30 different countries outside of the U.S. (at http://www.outofcopyright.eu/). The Open Knowledge Foundation has been encouraging the development of public domain calculators for many countries: see http://publicdomain.okfn.org/calculators/. See also Library of Congress Copyright Office. Circular 15a, Duration of Copyright: Provisions of the Law Dealing with the Length of Copyright Protection ( Washington, D.C. : Library of Congress, 2004) <http://www.copyright.gov/circs/circ15a.pdf>. Further information on copyright duration is found in Chapter 3, "Duration and Ownership of Copyright," in Copyright and Cultural Institutions: Guidelines for Digitization for U.S. Libraries, Archives, and Museums, by Peter B. Hirtle, Emily Hudson, and Andrew T. Kenyon (Ithaca, NY: Cornell University Library, 2009) available for purchase at http://bookstore.library.cornell.edu/ and as a free download at http://ecommons.cornell.edu/handle/1813/14142 .
Now, back to the issue of the letters. The key here is publication. I have just recently concluded a series of posts on exactly that subject. See "What Constitutes Publication in a Copyright Case -- Part Two" and the reference to Part One in that post. I am not going to go back through all the issues discussed in these two previous posts, but in referring to the Cornell Chart, it should be noted that for unpublished works, the present copyright term is the life of the author, plus 70 years for works from authors who died before 1945. If the death date of the author is unknown, then the copyright is extended to 120 years from the date of creation.
Ownership of the copyright passes to the authors' heirs just as any other property right. So, the owner or owners of the copyright to the letters are the legally determined heirs of the author or authors. Each author owns the copyright to his or her own letters.
If this sounds simple, you really need to read my previous posts on publication and some of the other articles I have referenced. It is not simple.