The theme seems to be religion and exploitation. Throughout the book, a succession of Martian societies realize that their long-cherished beliefs of the afterlife are in fact cruel hoaxes perpetrated on them by other societies, by self-serving rulers, or tragic misunderstandings. You'd think that some of them would catch on, and not just go around saying, "How foolish the outsiders are to believe that the Mysterious River Iss leads to paradise, rather than carnivorous plant-men, dreaded white apes, and a lifetime of enslavement! How lucky we are to know that true paradise is available at the end of the Long Ominous Tunnel From Which Nobody Returns!"

The racial obsessions are very 1913 (in that they assume that race is quite important), but not mean-spirited. Mars is peopled by different species and races, and it's assumed that each race will have its own "character" and its own monoculture, and that some kind of competition will exist. Burroughs doesn't seem to buy into the "purity" doctrines of contemporary racists, though: the Red Martians are portrayed as being admirable because they descend from all the other races, not because of any innate superiority. Members of all martian races are portrayed as being heroic and villainous by turns, though admittedly only John Carter is portrayed as having enough brains to know a crock of bs when he sees it. It's stupid, but not quite as ugly as some of the junk Burroughs did in Tarzan.

The best reason to read Burroughs is still IMO the prose style. For example, suppose you've just found out that the object of a recent crush is in an exclusive relationship with someone else. There's taking it gracefully, there's taking it badly, and then there's this:

"Dog," she hissed. "Dog of a blasphemer! [1] Think you that Phaidor, daughter of Matai Shang, supplicates? She commands. What to her is our puny outer world passion for the vile creature you chose in your other life?"
"Phaidor has glorified you with her love, and you have spurned her. Ten thousand unthinkably atrocious deaths could not atone for the affront that you have put upon me. The thing that you call Dejah Thoris will die the most horrible of them all. You have sealed the warning for her doom."
"And you! You shall be the meanest slave in the service of the goddess you have attempted to humiliate. Tortures and ignominies shall be heaped upon you until you grovel at my feet asking the boon of death."
"In my gracious generosity I shall at length grant your prayer, and from the high balcony of the Golden Cliffs I shall watch the great white apes tear you asunder."

[1] On reflection, I can't recall that there are actually any dogs on Mars at all. Perhaps this is meant to be a translator's liberty, and she really called him a "small-contemptible-six-legged-scavenging-pack-animal of a blasphemer."

They sure don't write 'em like that any more.

This is a series of three books about a romance between two artists who become aware of each other through weird supernatural means. The novelty here is mainly the format: the pages hold a series of postcards, intermixed with envelopes holding one-page letters you can remove from the books and read.

I didn't really get into these books. I think that the characters are supposed to be deep and soulful by authorial fiat, but I can't really see deep and soulful people conducting the long-distance romance of their life over so many postcards and one-page letters.  Also, the final resolution to the series didn't seem to answer many of the mysteries that had come up over the course of the books, but seemed a little arbitrary.

In the end, I couldn't help feeling I'd missed something, like if I read the whole series again I might find out unequivocally which of the events in the letters really happened, or that all the characters were secretly telepathic goldfish from pluto, or something like that.  Still, the artwork and overall design were great.