Question: Toni Lopopolo is a literary agent whose book publishing career began in 1970 in the publicity department of Bantam Books, where she helped publicize authors such as which English author of romance novels, one of the best-selling authors as well as one of the most prolific and commercially successful worldwide of the twentieth century, with her 723 novels were translated into 38 languages and she continues to be referenced in the "Guinness World Records" for the most novels published in a single year in 1976?

<P0-S0> Some writers have had prolific careers with hundreds of their works being published. <P0-S1>  While some best-selling authors have written a small number of books that have sold millions of copies, others have had lengthy careers and maintained a high level of output year after year. <P0-S2>  Dame Agatha Christie, the most-published novelist in history, is estimated to have sold 4 billion books, having written 69 novels and 19 plays. <P0-S3>  Her works were published between 1920 and 1976, equating to around three publications every two years. <P0-S4>  Dame Barbara Cartland has also sold millions of copies of her books but wrote many more than Christie. <P0-S5>  She spent 80 years as a novelist with 722 books published, averaging one book released every 40 days of her career. <P0-S6>  While Cartland wrote a significant number of full-length novels, other authors have been published many more times but have specialised in short stories. <P0-S7>  Spanish author Corín Tellado wrote over 5,000 novellas, selling 400 million copies of her books. <SEP> <P1-S0> According to "Guinness World Records", "White Christmas" (1942) by Bing Crosby is the best-selling single worldwide, with estimated sales of over 50 million copies. <P1-S1>  The song recognized as "the best-selling single of all time" was released before the pop/rock singles-chart era and "was listed as the world's best-selling single in the first-ever "Guinness Book of Records" (published in 1955) and—remarkably—still retains the title more than 50 years later." <P1-S2>  "Guinness World Records" states that double A-side charity single "Candle in the Wind 1997"/"Something About the Way You Look Tonight" (1997) by Elton John, a tribute to the late Diana, Princess of Wales, is "the biggest-selling single since UK and US singles charts began in the 1950s, having accumulated worldwide sales of 33 million copies," making it the second-best-selling single of all time. <SEP> <P2-S0> Toni Lopopolo is a literary agent whose book publishing career began in 1970 in the publicity department of Bantam Books, where she helped publicize authors such as Philip Roth, Barbara Cartland, Isaac Asimov and Louis L'Amour. <SEP> <P3-S0> Dame Mary Barbara Hamilton Cartland DBE CStJ (9 July 1901 – 21 May 2000) was an English author of romance novels, one of the best-selling authors as well as one of the most prolific and commercially successful worldwide of the twentieth century. <P3-S1>  Her 723 novels were translated into 38 languages and she continues to be referenced in the "Guinness World Records" for the most novels published in a single year in 1976. <P3-S2>  As Barbara Cartland she is known for her numerous romantic novels but she also wrote under her married name of Barbara McCorquodale and briefly under the pseudonym of Marcus Belfry. <P3-S3>  She wrote more than 700 books, as well as plays, music, verse, drama, magazine articles and operetta, and was a prominent philanthropist. <P3-S4>  She reportedly sold more than 750 million copies. <P3-S5>  Other sources estimate her book sales at more than two billion copies. <P3-S6>  She specialised in 19th-century Victorian era pure romance. <P3-S7>  Her novels all featured portrait-style artwork, particularly the cover art, usually designed by Frances Marshall.