The Warlord of Mars was published in 1914 and resolves the cliffhanger from The Gods of Mars. John Carter spends the entire novel trying to rescue Dejah Thoris from her imprisonment in the Temple of the Sun, where she has been sealed inside for a Martian year by the white Thern princess Phaidor. The chase takes Carter across nearly the full geography of Barsoom, from the polar regions to the northern continent of Okar, populated by the yellow Martian race no previous book had explored.
The book contains some of Burroughs’s most famous set-pieces: the long ice journey to Kadabra, the duels in the arena of the yellow city, the final confrontation with Salensus Oll, the jeddak of the yellow Martians. By the end Carter has united the various Martian races under his own leadership and earned the title Warlord of Mars. This trilogy with A Princess of Mars and The Gods of Mars forms the core of the Barsoom canon. Read all three together if you can.