Unpublished works pose particular problems, especially for archives. A quirk of copyright law means that many of these works, including some very old documents are in copyright until 2039.
Before the Copyright, Designs and Patents Act 1988 came into force on 1st August 1989 the situation was even more restrictive. Some unpublished works enjoyed perpetual copyright. The CDPA redefined the duration of copyright for those works to 50 years after the new Act came into force, in other words the end of 2039.
Among the works affected are previously unpublished works of known authorship created before 1st August 1989, (except pre June 1957 photographs), in all cases where the author died before 1969. The letters of a prominent scientist, written prior to 1989 and never published would be in copyright until 2039.
It follows that if we wish to reproduce any of those letters, we need permission from the copyright holder, who could for example be a descendant of the author. This situation affects the use of extensive material held by the National Archives and similarly restrictive rules apply to some unpublished material which is Crown Copyright.
Could there be change on the horizon? The Government has given itself powers via the Enterprise and Regulatory Reform Act 2013 to amend this particular quirk of copyright duration. The result will be that some unpublished copyright works will move into the public domain while others will have a shorter term. We await the Statutory Instruments needed to implement the changes.