The details are a little fuzzy after the passage of 500 years, but Sir Thomas Malory underwent a rather abrupt career change. Most of what’s known about him as author comes from the back of the Caxton printing of Le Morte d’Arthur, in that he was a “knight prisoner”. We know something more of the prisoner thought to be him. The man in Newgate Prison was allegedly quite the rapscallion.
This fellow, Sir Thomas Malory of Newbold Revel, was accused of multiple crimes: theft, rape (the term at the time included consensual sex with a woman whose husband hadn’t agreed), and various banditries committed in 1450-51. Since this was at a turbulent time during the Wars of the Roses and the crimes were all against the Lancastrian side, Malory (or Malleore, or Mallere) may have thought he was waging war.
As may be, he was too slick to hold (he swam the moat at Maxfield Castle to get away –ick!), escaping numerous times to raid again. He did spend close to ten years in Newgate, not far from the Greyfriars monatery, whose library he was allowed to use. While there, he translated, compiled, and edited many of the French and Middle English Arthurian stories. The Gareth story contains a lot of his original work.
William Caxton published this work in 1485 as Le Morte d’Arthur. It went through several printings before the English Civil War. Two copies of the original issue exist in major libraries, and perhaps there is a treasure lurking on the shelves of some bibliophile or in an attic in a box.
Malory never saw the book in print during his lifetime. “In print” wasn’t even a thought–he’d been dead five years when Caxton brought the first moveable type printing press to England.
He’d probably be amused that the world still reads the book he compiled during his imprisonment. He’s probably be really amused at the changes in technology we use to read it, and by the endless adaptations of his work. He was amusing himself while sitting on his butt in prison.
I’d say he used the time wisely.