The Structure Of A Series: John Carter

I never got into Tarzan. The character teaching himself to read English from a picture book is impossible and should never have been published. Knowing without being told that the sound "Tarzan" corresponds to the written letters T, A etc is also impossible. The sequence of events, not a plot, in Tarzan's Quest was so implausible and chaotic that I found it the opposite of pleasurable to read it. Tarzan At The Earth's Core I read only because it was a Tarzan/Earth's Core crossover. Since there are over twenty Tarzan volumes, I have read very little of this series.

By contrast, Edgar Rice Burroughs' Martian series had something going for it. The New English Library republished the eleven volumes annually through my teens so I bought each as it appeared:

an opening trilogy in which Carter saves the lives of everyone on Mars/Barsoom, overthrows the religion of Issus and becomes Warlord of Mars;

one book each about Carter's son, his daughter and a second Earthman transported to Mars;

two Martian warriors' stories transmitted to Earth by the second Earthman;

a return to Carter who now has a granddaughter, fights the Assassins' Guild and visits a Martian moon and Jupiter.

There are three sequel series:

Carson Napier aims for Mars but reaches Venus;

like John Carter and Ulysses Paxton, a third soldier dies on Earth and is projected across space in an astral body that solidifies on arrival - in this case not on Mars but on an extrasolar planet atmospherically linked to several others in a common orbit (a precursor of Larry Niven's Smoke Ring) so that he is able to make an interplanetary crossing by airplane (Beyond The Farthest Star);

in the future, Earthmen and Martians communicate by radio and each planet launches a spaceship towards the other but the Earth ship, the "Barsoom," crashes on the Moon which then invades Earth (the Moon Maid trilogy).

Thus, Burroughs wrote an increasingly intricate sequence of imaginative interplanetary stories -

Two trilogies (almost) recount Carter's adventures:

the opening volumes are a structural trilogy with cliff hanger endings to volumes I and II and a culmination ("...let him be Jeddak of Jeddaks, Warlord of Barsoom!") at the end of volume III;

three later volumes, though not structurally a trilogy, are the welcome return of Carter, soldier, statesman, scientist and scholar, as both narrator and central character, the best way that ERB could have completed the series.

Two heroines are title characters of opening volumes, A Princess Of Mars and The Moon Maid, although the second installment of the Moon trilogy presents a different kind of story, describing an oppressive society.

Two Earthmen go to Mars.

Two children of Carter adventure on Mars.

Two worlds, Mars/Barsoom and the Earth's Core/Pellucidar, are contacted on the Gridley Wave.

Two Martian warriors' stories are transmitted to Earth.

Two other worlds are visited by Carter. (A Martian moon is small but a spaceship and its occupants shrink as they approach so that, when they are on it, it is as large to them as Barsoom with proportionate inhabitants and surface features!)

Two Earth people go to Venus. (A later volume discloses a second mysterious case, more akin to Carter's astral travel.)

Two (or three) planets receive unexplained astral projections from Earth.

Two spaceships launched towards Mars arrive elsewhere:

the first, deflected by the Moon, reaches Venus;
the second, sabotaged by a crew member, lands on the Moon.

Two moons, terrestrial and Martian, are inhabited.

Two worlds, Earth and its Moon, are hollow spheres with inhabited interiors.

Two worlds, the enclosed Pellucidar and the clouded Venus, are believed by their inhabitants to be bowl-shaped and floating on an ocean. (It would have made more sense for the Pellucidarians to believe that the universe was endless solid rock with many other worlds occupying empty spaces within it and tunneling machines the equivalent of spaceships.) 

Two interplanetary invasions are planned:

Jovians will invade Mars, then other planets;
Moon Men do invade Earth where they rule tyrannically for several generations.

Two outcomes are unknown: of the Martian-Jovian conflict and the extrasolar interplanetary crossing.

Four planets, Mars, Venus, Jupiter and extrasolar Poloda, are visited and we are told that there is a Mercurian civilization.

ERB stays at the still center of the storm, receiving visits and manuscripts from Carter, radio messages from Paxton, telepathic messages from Napier, telekinetic messages from Poloda and accounts of a pre-remembered future from Julian. He does get some action on a future polar bear hunt in 1969.   

In HG Wells' inhabited Solar System, we do not know the outcome of the Martian invasion of Venus because Wells deliberately left this, and the future of life in the universe, uncertain. In the ERBian universe, ERB would simply have continued to add extra installments if he had lived longer. However, he did leave some mysteries unsolved, the strangest being the origin of John Carter who remembers no childhood but has always, in his memory, been an adult. Very near the end of the series, he speculates just once that he might be the materialization of a long dead warrior...so, potentially, another series remains to be written.