One might reasonably conclude that the first three books of the Barsoomian series were intended to represent a single, complete saga. The first two tales, A Princess of Mars and The Gods of Mars, had both ended with classic cliffhangers, the former with John Carter's loss of consciousness at the Atmosphere Plant and the latter with the imprisonment and possible death by stabbing of Dejah Thoris at the Martian south pole.
A single hero and heroine were featured in the three books, while about them Burroughs built his marvelously complete picture of Barsoom, its monstrous fifteen-foot tall green barbarians, its various civilized races of dominant red men, admirable though isolated yellow and black men, and nearly extinct whites, its ancient culture and the limited survival of that culture into later times.
With the romance of John Carter and Dejah Thoris as a continuing theme pervading the adventures of the Earth man, the main focus of the three books is upon the latter's fantastic rise in the society of Mars: from a literally naked and weaponless newcomer, unable even to speak the language of the planet, rising to the Warlordship of all Mars. The question then arises, What do you do for an encore?
If Burroughs had dropped his Martian series with the completion of the trilogy, I suspect that those books would hold an even higher place in the general field of science fiction than they do (as might the earliest Tarzan books if there were not the numerous later pot-boilers of that series to dim their lustre). A similar case was that of Burroughs' contemporary pulp writer, George Allan England.
[A discovery of Munsey publications editor Robert H. Davis, England broke into the Munsey Cavalier magazine in 1912 with The Vacant World, a novel of the future in which a natural disaster destroyed civilization and very nearly annihilated mankind. Two survivors, an engineer and his secretary,