At the Earth's Core - Plain text version at Project Gutenberg. - http://www.gutenberg.org/etext/545
Pellucidar - E-text at Project Gutenberg. - http://www.gutenberg.org/etext/605
Page By Page Books: At the Earth's Core - Chapter-indexed and paged HTML text of the novel. - http://www.pagebypagebooks.com/Edgar_Rice_Burroughs/At_the_Earths_Core/