Fond though I am of the Ballantine Adult Fantasy covers, I must admit that most come off as amateurish; here, obviously, we are in the presence of a master. Being a geek, however, I must note that, like most Burroughs illustrators, Frazetta takes some liberties with the green Martians, making them look more human than Burroughs describes them.
Their eyes were set at the extreme sides of their heads a trifle above the center and protruded in such a manner that they could be directed either forward or back and also independently of each other, thus permitting this queer animal to look in any direction, or in two directions at once, without the necessity of turning the head. [A Princess of Mars]On the other hand, those are some pretty awesome apes. It saddens me that I haven't yet come across a copy of A Princess of Mars in this line. However, as a consolation I do have the Nelson Doubleday book club edition (1972) of Thuvia, Maid of Mars / The Chessmen of Mars.
Then we have the 1974 edition of Swords of Mars / Synthetic Men of Mars:
Since we're on Frank Frazetta and Edgar Rice Burroughs, here's the cover to an Ace edition of At the Earth's Core:
* I have something like four editions of most of the Mars books. And I'm likely to acquire more if no one stops me. Yes, I need help.