Letters from the Earth is one of Mark Twain's posthumously published works. The essays were written during a difficult time in Twain's life; he was deep in debt and had lost his wife and one of his daughters. The content concerns morality and religion and strikes a sarcastic (Twain's term throughout the book) tone. Initially, his daughter, Clara Clemens, objected to its publication in March 1939, probably because of its controversial and iconoclastic views on religion, claiming it presented a "distorted" view of her father. Henry Nash Smith helped change her position in 1960.

Clara explained her change of heart in 1962 saying that "Mark Twain belonged to the world" and that public opinion had become more tolerant. She was also influenced to release the papers due to her annoyance with Soviet propaganda charges that her father's ideas were being suppressed in the United States. The papers were selected, edited and sequenced for the book in 1939 by Bernard DeVoto.

The book consists of a series of commentaries in essay and short story form. Many of these pieces express Twain's discomfort with and disdain for Christianity, both as a theological position and a lifestyle. The title story consists of eleven letters written by the archangel Satan to archangels Gabriel and Michael, about his observations on the curious proceedings of earthly life and the nature of man's religions. Other pieces in the book include a morality tale told as a bedtime discussion with Twain's children, Susy and Clara, about a family of cats, and an essay explaining why an anaconda is morally superior to Man.

Textual references make clear that sections, at least, of 'Letters from the Earth' were written shortly before his death in April 1910. (For instance, Letter VII, in discussing the ravages of hookworm, refers to the $1,000,000 gift of John D. Rockefeller Jr. to help eradicate the disease – a gift that was announced on October 28, 1909, less than six months before Twain's death.) However, another excerpt appears in a discussion of the Palestinian town of Nablous, in The Innocents Abroad. This passage was written more than four decades before his death, in 1867 or 1868, and appears to be an oblique reference to the idea that later became 'Letters from the Earth.'