Letters From the Earth
Letters From the Earth was composed over a long period of time, from his first travels overseas in the Holy Land, as reported in The Innocents Abroad, to within six months of his death. The material was edited by Bernard DeVoto in 1939 but was not actually published until 1962 when his daughter Clara gave permission to do so. The version I (or SLClemens) read was published on http://www.positiveatheism.org/hist/twainlfe.htm
The work is quite controversial and does contain material of a sexual nature but not such as I would consider to be "Adult" but it is meant for mature, meaning intelligent, audiences. Audiences that are not offended by challenges to their religious views. Challenges to what they may deem to be Man's "Moral Sense".
God, as envisioned by Twain, is a creator but takes no further interest in his creation. The universe God creates follows "Natural Law" and needs no further guidance. The god Satan reports on in these letters is a jealous god and one that is constantly meddling in the world's affairs and is quite the hypocrite. If this bothers you, don't listen to the attached audio files.
Prelude: The Creation