A Princess of Mars

Before many of the current genres were cemented, Edgar Rice Burroughs began putting out a seemingly endless blur of sci-fi/fantasy/pulp fiction books, starting with A Princess of Mars. He would later become super famous for his Tarzan series, but the Martian books are still great reads. It is a story set on the dying planet Mars, and its hero is John Carter, ex-Civil War vet - it's full of heroic, non-PC, early 20th century manliness.

Carter has been taken prisoner by the Green Men of Thark. And the titular princess in the story is Dejah Thoris, who leads a clan on Mars. The princess and Carter must free themselves in time to save the planet from destruction as its Atmosphere Factory begins to shut down. A Princess of Mars inspired many modern science fiction writers, including Ray Bradbury, John Norman and Arthur C. Clarke, and influenced the imagination of distinguished scientist Carl Sagan, who read the book as a child.