Some writers have had prolific careers with hundreds of their works being published. 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. Dame Agatha Christie, the most published novelist in history, is estimated to have sold 4 billion books, having written 69 novels and 19 plays. Her works were published between 1920 and 1976, equating to around three publications every two years. Dame Barbara Cartland has also sold millions of copies of her books but wrote many more than Christie. She spent 80 years as a novelist with 722 books published, averaging one book released every 40 days of her career. While Cartland wrote a significant number of full-length novels, other authors have been published many more times but have specialised in short stories. Spanish author Corín Tellado wrote over 4,000 novellas, selling 400 million copies of her books.