From time to time the SF Encyclopedia team is asked about rights and copyright issues. We are not qualified to offer legally bullet-proof options on this complex subject. Fiction and nonfiction can generally be safely assumed to be in the public domain when 70 years have passed since the death of its author. Even this rule of thumb varies from country to country: the public domain library at Project Gutenberg Australia contains works which cannot be included in the main (US-based) Project Gutenberg. There are also surprising exceptions resulting from hiccups in US copyright legislation. For example, it used to be necessary to renew copyright in magazine stories in order to retain protection, and some stories by major authors slipped into the public domain as a result of non-renewal. Project Gutenberg quite legitimately hosts several by Philip K Dick.
However, even when copyright has lapsed, some lucrative genre properties are further protected by trademark registration – which can continue indefinitely if regularly renewed by the owner. For example, though Edgar Rice Burroughs died in 1950, it would be unsafe to assume that anyone can then make free with his Tarzan character from 2020 onwards: many uses of the Tarzan name are covered by vigorously defended trademarks, as noted here.
For works in print, the first recourse should always be to the author (are they alive? do they have a website listed in their SFE entry or the Internet Speculative Fiction Database?) or the rights department of the relevant publisher.
Here are some links which may be of use:
This resource page maintained by the Science Fiction Writers of America has information on rights and representation for a number of authors, plus a link to the more general information page below.
This includes a link to Stanford's Copyright Renewal Database, through which you can check whether copyrights have lapsed or were renewed.
This front page links to the copyright search page at the Library of Congress site.
Project Gutenberg actively researches the copyright status of the public domain works hosted at the above-linked site, so it should be safe to assume that Gutenberg titles are out of copyright. This is unfortunately not the case for many other online resources – not merely overt pirate sites but the highly respectable archive.org, which preserves the contents of many websites whose owners failed to acquire rights to publish copyright material online.
See also Project Gutenberg Australia.