By the way, please keep in mind that this series was written at the turn of the 20th century and is set just after the Civil War.
- Princess of Mars also as an audiobook
- The Gods of Mars
- Warlord of Mars
- Thuvia, Maid of Mars
- Chessmen of Mars
I haven't been able to find books 6-11 for free, but I will keep looking. In the meantime, you can read it online, with illustrations at read.gov
A little digging around the interwebz helped me find the rest of the series. They are zip,text, and HTML versions at Project Gutenberg Australia. I link to the text versions here.
- The Master Mind of Mars
- A Fighting Man of Mars
- Swords of Mars
- Synthetic Men of Mars
- Llana of Gathol
- John Carter and the Giant of Mars
- Skeleton Men of Jupiter