Edgar Rice Burroughs returned to Barsoom in 1913 to write The Warlord of Mars. Continuing from the cliffhanger ending of Gods of Mars, the story was serialized in the pages of the All-Story Magazine incarnation of Argosy, and later published as a novel in 1919.
Dejah Thoris and Thuvia of Ptarth are still imprisoned in a strange prison within the bowels of the Temple of the Sun with the treacherous Princess of the Holy Therns, Phaidor.
Not one to simply sit and wait out the year before the door opens again, John tries in vain to find a way in, and comes across a conspiracy between Thurid, a Black Martian of the First Born who bears a deep grudge against Carter since an embarrassing defeat in Gods of Mars and Maitai Sheng, leader of the White Martians and high priest of the cannibalistic Holy Therns.
The conspirators open the prison and spirit the women away, with John Carter and his trusty calot Woola giving chase, first to the equatorial jungle on the other side of the planet where the Red Martian city of Kaol holds sway, and then to the frozen north, where an entire race of Yellow Martians has been locked away from the rest of Barsoom by natural fortifications, and the cruel Salensus Oll rules as Jeddak of Jeddaks.
This feels like a shorter story than the previous two, but that could be the rocket-fast pace of the chase that takes up the bulk of the book, culminating in a titanic battle for the city at the North Pole of Mars, where every ally that John Carter has ever made comes to his aid. Mighty Tars Tarkas of the Greens, Thuvia the Princess of Ptarth who can sooth wild animals with her voice, Thuvan Dihn, Lord of Ptarth who has been searching for his daughter, Xodar the new chief of the Black Martians, Carthoris the son of John Carter and Dejah Thoris, Kantos Kan of Helium, Tardos Mors and Mors Kajak, the lost lords of Helium (and Dejah's father and grandfather), and Talu of the Yellow Martians. There are even some Therns who have allied with the forces of Helium who show up.
Much of the second half of the book is wrapping up story threads and giving satisfying conclusions, but the most satisfying development comes in how Phaidor grows as a character by the end in a touching scene.
Then the final chapter is a victory lap as John Carter makes his way back to Helium with Dejah Thoris where he is then put on trial for his heresy in the Valley Dor. Its a fake out, of course, and the statute of limitations on spoilers is long gone on a story from a century ago, so I'll tell you that John Carter is crowned Jeddak of Jeddaks by a unanimous vote of all the people he's forged together into his alliance over the past three books and it ends with John taking Dejah into his arms and kissing her.
Its about as FLAWLESS VICTORY as you can get in fiction, and it is completely and truly earned. John Carter has gone through hell and back across an entire planet, fighting anything and anyone who would stand in his way. He's been captured, tortured, thrown off of multiple buildings, thrown off of Mars entirely, and always kept coming back. Not for conquest, though he does that and ends centuries of racial hatred along the way, but for love.
Its a deeply satisfying conclusion to the initial trilogy and it hints at future adventures among the next generation of heroes, with Carthoris and Thuvia drawing close to each other. Indeed, there would be 8 more books in the Barsoom series.
Absolutely recommended. If you want to raise good boys, give them Harry Potter. If you want to raise good men, give them John Carter.