Historical and children’s fiction author Rosemary Sutcliff wrote a book for adults (as opposed to children) about King Arthur – Sword at Sunset – a best seller in the UK in 1963. She said twenty years later:
I had determined from the time that I was very young that there was a real person there, and that I would love to find and reconstruct that person. […] Most of the actual research I did for the book (Sword at Sunset), apart from knowing the Arthurian story from the romance versions, was into Dark Age life and history as far as they were known. Then I worked into this setting the Arthur who seemed to me to carry weight, to be the most likely kind of person. It was very strange because I have never written a book which was so possessive. It was extraordinary–almost frightening. […] I would take the book to bed with me at night, and work there until I dropped off to sleep about two o’clock in the morning, too tired to see any more. Then I would wake up about six o’clock, still thinking about it. It was addictive. It was almost like having the story fed through to me, at times. I do my writing usually in three drafts, and I would go from the first to the second draft, from the second to the third, and find bits of the book that I had no recollection of having written at all.
Source: From Raymond H. Thompson’s interview with Rosemary Sutcliff in August 1986