Here we go...
The Copyright Term Extension Act of 1998 retroactively lengthened copyright by twenty years
for books copyrighted after January 1, 1923.
Unfortunately, the copyright status of books published in the 20th century
is complicated by legislation that has extended copyright eleven times during the last fifty years.
Until a congressional act of 1992, rightsholders had to renew their copyrights. The 1992 act removed
that requirement for books published between 1964 and 1977, when, according to the Copyright Act of 1976, their copyrights
would last for the author’s life plus fifty years.
The act of 1998 extended that protection to the author’s life plus seventy years. Therefore,
all books published after 1963 remain in copyright, and an unknown number– unknown owing to inadequate information about
the deaths of authors and the owners of copyright– published between 1923 and 1964 are protected by copyright.
To dig deeper, find this reference:
"Restricting Access to Books on the Internet: Some Unanticipated Effects of U.S. Copyright Legislation,"
by Paul A. David and Jared Rubin, in the "Review of Economic Research on Copyright Issues, Vol.
5, No. 1 (2008)