As a followup to the recent dustup in which a group of copyright trolls who claim to control the rights to all of the Conan stories (even the out-of-copyright ones!) shut down Broken Sea Audio's distribution of free audiobooks based on the public domain stories, here's a great, exhaustively researched article on the copyright status of the Conan stories, written by a fan:
Many of the works of REH were first published during or shortly after his lifetime, from 1922 through 1939. More came out over the decades that followed, with a large amount seeing first publication after 1964. Under US law, all of the REH works first published prior to 1964 were subject to the registration, renewal and notice requirements of the 1909 Copyright Act ("the 1909 Act"). Under the 1909 Act, copyright was not automatically applied to a published work, as it is under the current Act. Instead, to obtain copyright, the work had to be first published subject to a number of rules. These included proper notice affixed to the work, and prompt registration. If works were published without meeting these formalities, such works were usually injected into the Public Domain ("the PD"). Further, 28 years after publication there was a one year window in which certain classes of people or entities could file for a renewal of the copyright for an additional 28 year term (later extended by Congress to a total term of currently 95 years). In practice, the courts have said that as long as the original registration is filed prior or simultaneously with the renewal, the registration was still valid. Further, the courts have on occasion been forgiving of flawed but still present notice under the 1909 Act. But, the courts have been quite strict about the one year window for renewals. Complete lack of notice also generally automatically injected the work into the Public Domain, though the totality of the circumstances can affect that issue.