The entire collection of Mark Twain books and a list of related external links on Mark Twain can be found here: http://bookyards.blogspot.com/2008/07/mark-twain-ebooks-and-resource-links.html
Free audio recording of Thoreau's famous essay on Civil Disobedience. This essay is said to have been an influence on Ghandi and Martin Luther King. It's surprising how relevant much of the piece remains given the current political situation.
Librivox has over 300 totally free audio books in the public domain. Volunteers read classic works by authors such as Mark Twain, Jane Austen, Dickens, Shakespeare, Tolstoy, etc., etc., etc.
Wide range of Public Domain Audio books. Copyright status is subject to individual country law. Open source project that aims to create audio books of any available public domain books