The story picks up roughly 10 years after the first, with the legendary John Carter back on Mars, hoping to reunite with his wife, Dejah Thoris. When he lands on mars, though, he does so in the place that no one can leave: Valley Dor, the Barsoomian afterlife. That’s hardly the type of thing that would stop a man like Carter, though, and he has but a single pervading desire throughout the novel: rescue his wife.
The Gods of Mars was first serialized over the course of five months before finally being edited into a book. Burroughs makes himself a character in the book, while the manuscript is written by John Carter himself. The novel combines a litany of genres; it is obviously science fiction, with a significant amount of planetary romance thrown in. In parts, it resembles a traditional sword and sorcery, but with elements of science. The Barsoomian series can be credited with the popularity of the planetary romance genre, despite not being the first.
Race and religion are both important themes of the story. There are clear divisions with the different colored Martian races; meanwhile, he depicts religion as something that can succumb to evil and abuse (especially when it is cultist).
If you prefer a physical copy, you can get one here. If you want a collection of the entire series, you can get that here as well. Meanwhile, the Mars Trilogy has the first three novels in the Barsoom series.