Well the copyright law in effect when he died would have allowed 28 years from publication but the one in effect today is just messy and i honestly can't tell if this is a public domain work or not. Look at this
http://www.law.cornell.edu/uscode/17/303.html
(a) Copyright in a work created before January 1, 1978, but not theretofore in the public domain or copyrighted, subsists from January 1, 1978, and endures for the term provided by section 302. In no case, however, shall the term of copyright in such a work expire before December 31, 2002; and, if the work is published on or before December 31, 2002, the term of copyright shall not expire before December 31, 2047.
(b) The distribution before January 1, 1978, of a phonorecord shall not for any purpose constitute a publication of the musical work embodied therein.
yet at the same time
http://www.law.cornell.edu/uscode/ht...2----000-.html
(c) Anonymous Works, Pseudonymous Works, and Works Made for Hire.— In the case of an anonymous work, a pseudonymous work, or a work made for hire, the copyright endures for a term of 95 years from the year of its first publication, or a term of 120 years from the year of its creation, whichever expires first. If, before the end of such term, the identity of one or more of the authors of an anonymous or pseudonymous work is revealed in the records of a registration made for that work under subsections (a) or (d) of section 408, or in the records provided by this subsection, the copyright in the work endures for the term specified by subsection (a) or (b), based on the life of the author or authors whose identity has been revealed. Any person having an interest in the copyright in an anonymous or pseudonymous work may at any time record, in records to be maintained by the Copyright Office for that purpose, a statement identifying one or more authors of the work; the statement shall also identify the person filing it, the nature of that person’s interest, the source of the information recorded, and the particular work affected, and shall comply in form and content with requirements that the Register of Copyrights shall prescribe by regulation.
(d) Records Relating to Death of Authors.— Any person having an interest in a copyright may at any time record in the Copyright Office a statement of the date of death of the author of the copyrighted work, or a statement that the author is still living on a particular date. The statement shall identify the person filing it, the nature of that person’s interest, and the source of the information recorded, and shall comply in form and content with requirements that the Register of Copyrights shall prescribe by regulation. The Register shall maintain current records of information relating to the death of authors of copyrighted works, based on such recorded statements and, to the extent the Register considers practicable, on data contained in any of the records of the Copyright Office or in other reference sources.
(e) Presumption as to Author’s Death.— After a period of 95 years from the year of first publication of a work, or a period of 120 years from the year of its creation, whichever expires first, any person who obtains from the Copyright Office a certified report that the records provided by subsection (d) disclose nothing to indicate that the author of the work is living, or died less than 70 years before, is entitled to the benefits of a presumption that the author has been dead for at least 70 years. Reliance in good faith upon this presumption shall be a complete defense to any action for infringement under this title.
And how are we going to decide published if the papers were made available to scholars to read and quote before the full text was put in book form? And is the name attached Mark twain or Samuel Clemens it seems to matter for copyright term. This one is messy making it perfect for debate here.