Literature of the 20th century refers to world literature produced during 1900 through the 1990s. The best-selling works of the 20th century are thought to be "Quotations from Chairman Mao" (1966, 900 million copies), "Harry Potter and the Philosopher's Stone" (1997, 120 million copies), "And Then There Were None" (1939, 115 million copies) and "The Lord of the Rings" (1954/55, 100 million copies). The Lord of the Rings was also voted "book of the century" in various surveys. "Perry Rhodan" (1961 to present) boasts as being the best-selling book series, with an estimated total of 1 billion copies sold.