Cadfael's stories are written by Ellis Peters.
She also wrote The Brothers of Gwynedd Quartet
which is the story of Llewelyn, first true Prince of Wales. She wrote it under the name Edith Pargeter. The quartet is comprised of: Sunrise in the West, The Dragon at Noonday, The Hounds of Sunset,
and Afterglow and Nightfall.
But as this is a GK Chesterson thread, the medievalists will have to wait for their own thread.