text: The best-selling works of the 20th century are estimated 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 ) .
simplified: 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 ) .