Robert E. Howard (the creator of Conan) committed suicide in 1936, leaving behind no family. Thus (depending on what country you live in) all of his books and stories are now public domain. (Any Conan stories written by other authors however belong to those writers. ie. "Conan the Berserker" by fantasy author Charles Moffat.)
We should note however that Public Domain laws vary from country to country... the following is a list of English speaking countries and the number of years required for copyright to run out.
Australia: 70 years after the death, or depending on the media it is 70 years from publication for sound recordings and films. For television broadcasts and sound broadcasts its 50 years after being broadcast. Photography has separate rules.
Canada: 50 years after the death of the creator.
New Zealand: 50 years after the death of the creator.
United States: 70 years after the death, but depending on when it was published or if the creator published anonymously then it is 95 years.
United Kingdom: 70 years after the death or 50 years after released/created for sound recordings.
There is also the Universal Copyright Convention...
Life + 25 years for general works.
25 years from publication (multiple creators).
10 years for photographic works or artworks.
Now depending on what the topic is you may also have to deal with film rights (due to previous films made on the same topic).
For example J.K. Rowling (of Harry Potter fame) did not actually create Harry Potter. Harry Potter is actually a character from the 1986 film "Troll" and was created by John Carl Buechler and Ed Naha. The makers of Troll have actually decided to remake Troll with a bigger budget, starring (guess who?)... Harry Potter. And there is nothing J.K. Rowling can do about it because she ripped off the name Harry Potter in the first place. (She does however own the rights to the Harry Potter franchise and words like Hogwarts, Slitherin, etc.)
That was a mistake by Rowling and her producers to not obtain the rights to the Harry Potter name.
But what this does raise is an interesting matter... all the books and films which were either published 50-70 years ago, and all the writers who died approx. 70 years ago. Or in some cases, earlier.
If you do your research you will find there is a long list of movies from the 1930s, 1940s, 1950s and even the 1960s / 1970s that are now Public Domain. Examples
1925 - The Lost World
1937 - Dick Tracy
1939 - Gulliver's Travels
1943 - The Outlaw
1944 - Zorro's Black Whip
1955 - The Fast and the Furious
1959 - Plan 9 from Outer Space
1959 - The Giant Gila Monster
1971 - Lady Frankenstein
1978 - Planet of Dinosaurs
The list of films is actually quite long... and for books there are hundreds of thousands that are either Public Domain or will soon become Public Domain.
Because of that status it also means people can take books and re-write them, thus creating books like:
Pride and Prejudice and Zombies
Sense and Sensibility and Sea Monsters
There is a lot of money to be made selling / reproducing / recreating / changing Public Domain works. For writers it means an extra source of income and a way to get noticed by taking something old and making it new again.