The Gods of Mars. by Edgar Rice Burroughs

The Gods of Mars is Burroughs' sequel to A Princess of Mars. After ten long years, John Carter is again transported to Mars to try and determine the fate of his wife Dejah Thoris, but finds himself in the forbidden Valley Dor, from which no man may return. Published serially in five parts between January and May 1913, this sequel appeared a year after the initial serialization of its predecessor. It was eventually published in its full novel form in 1918.
Although the Martian series contains ten books in total, the first three-of which The Gods of Mars is the second-are often considered a stand-alone trilogy. Throughout the series, Burroughs' imagination and sense of adventure shine through, and his extravagant prose and innovative vocabulary raise the works up above run-of-the-mill pulp fiction.