Edgar Rice Burroughs wrote the Barsoom series. It inspired a number of well-known 20th century science fiction writers, including Ray Bradbury and Arthur C. Clarke, and was also inspirational for many scientists in the fields of space exploration and the search for extraterrestrial life, including Carl Sagan.
The first book in the trilogy is A Princess of Mars, a science fantasy novel full of swordplay and daring feats. It is considered a classic example of 20th century pulp fiction. It is also the beginning of the planetary romance, a sub-genre of science fantasy that became highly popular in the following decades. Its early chapters also contain elements of the Western. The story is set on Mars, imagined as a dying planet with a harsh desert environment.
The second book is The Gods of Mars, a 1918 science fiction novel. It is most closely related to the planetary romance genre, similar to sword and sorcery, but including scientific aspects. Planetary romances mostly take place on the surface of an alien world (Mars, in this case), frequently include sword fighting, monsters, supernatural elements such as telepathic abilities, and civilizations similar to Earth in pre-technological eras, particularly with the inclusion of kingdoms, empires or religious societies.
The third book is The Warlord of Mars a science fiction novel. Burroughs's vision of Mars was loosely inspired by astronomical speculation of the time, when Mars was seen as a formerly Earthlike world which was becoming less hospitable to life due to its advanced age, and its newly observed canals. In the novels, scarce water is distributed through these canals. In this world, the main character, John Carter, discovers fantastic creatures and ancient mysterious Martian races. The action crosses the planet from one civilization hidden in the southern polar ice-cap to another hidden in the northern polar ice-cap.