You're after the Barsoom series, which is all the Mars books. John Carter of Mars is the final book and contains the last two stories.
Here's an omnibus of all the Barsoom books. http://www.mediafire.com/?p77dpzid1kk1d7u
Another one that is well known by him is the Land that Time Forgot trilogy. Basically a land where cavemen are fighting dinos, much like Arthur Conan Doyle's The Lost World. http://www.mediafire.com/?atya5y51x9u93dk
You may want to also check out the Moon series. It is a three parter, originally written about the evil Communists, back when it wasn't cool to hate the commies. He had to change the setting from Russia to the moon, with the evil people being called "Kalkars" instead of Bolsheviks, in order to get it published. - http://www.mediafire.com/?ckp1e1mmw9jf61c
And the Venus series. This one is similar to the Barsoom books, with the main character actually trying to get to Mars originally. He winds up on Venus due to a miscalculation, and it is more of a waterworld than the desert he expected, and has to deal with the Amtor people. - http://www.mediafire.com/?vo3bss1m8xj8mdo
Arguably, his most famous work is the Tarzan series. This one, I have broken up into 5 separate ebook files.
Volume 1 - http://www.mediafire.com/download.php?kukfo9shkgqp11m
Volume 2 - http://www.mediafire.com/download.php?18v021lcjl3qg31
Volume 3 - http://www.mediafire.com/download.php?8xm8k3og0fxx64f
Volume 4 - http://www.mediafire.com/download.php?y22tyef44o42sho
Volume 5 - http://www.mediafire.com/download.php?ht9rqd0w0ew0rz0
And if you've read the Tarzan books, you may want to read his Pellucidar books, which has a Tarzan crossover. Those are basically about how the earth is hollow, and the things inside the earth. http://www.mediafire.com/download.php?0h1z5ov6dlj2mtc
In most of the world, these are public domain (he died in 1950). Unfortunately, due to Sonny Bono and his Copyright Term Extension Act, they're still copyrighted for another 8 years in the US. A guy I know named HarryT, compiled these, and so I'm giving credit where credit is due.