The Gods of Mars by Edgar Rice Burroughs (1918)
Captain John Carter of Virginia returns once again by inexplicable means to the planet Mars, but instead of appearing in the arms of his beloved wife Dejah Thoris he finds himself at the shores of some unknown sea and under attack by hordes of bizarre plant men and carnivorous white apes. And so he embarks on an adventure where he plumbs the mysteries of Mars's South Pole, discovers the secrets of the Therns and the central conspiracy behind the Martian religion.
If you had to come up with a tag line for this novel is would be, "Three times the action, three times the adventure, three times the princesses". Burroughs is obviously worried here about letting down his readers after the success of A Princess of Mars, so he hedges his bets by ladling on the action with a trowel. There are battles royal, fleets of air ships, rushing about, escapes, sword fights, skulkings, revolts, wars, intrigues, treacheries, revelations and just about anything else you can imagine. There are even not one, not two, but three sets of villains to keep the reader's attention.
Burroughs maintains much of what made the previous novel work. We're once again in a new world to explore, there's loads of action and he is able to convey that marvelous sense of the exotic that makes Burroughs truly stand out as a writer. The only real criticism here is that he sometimes puts it on too heavy and we come away wishing that he'd just get on with it.
Still, it is a cracking read with lots of swashbuckling and a cliffhanger ending, so we can be forgiving.