This book is based on William Caxton’s translation of 15th century French author Raoul Lefèvre book Histoire de Jason (he wrote in 1460).
The Histoire de Jason is known from 20 manuscripts and 30 different printed editions, and was translated in English in 1477 by William Caxton, and in Dutch in 1485. Lefèvre was the chaplain of Philip the Good, the creator of the Order of the Golden Fleece, which was based on the classical Jason story.