Looking around for Mars, I was quite please with the edition of
The Land That Time Forgot: The Complete Caspak Trilogy in One Volume from Ignacio Hills Press. This is my favorite Burroughs novel—and it is
one novel, serialized in three parts in
Blue Book originally and published as a single hardback; it was in the '60s paperback boom that the three parts of the serial were broken up into individuals novels,
The Land That Time Forgot,
The People That Time Forgot, and
Out of Time's Abyss. As a whole novel, to be read straight through, it brings many more rewards.
The ProjectGutenberg edition and the free version on Amazon aren't well-formatted, so I think 99˘ is a great bargain for the Ignacio Kindle version, which includes some of the illustrations.
http://www.amazon.com/Land-That-Time-Forgot-ebook/dp/B001ECQKU2/ref=sr_1_2?s=digital-text&ie=UTF8&qid=1326969265&sr=1-2