I read this as a teenager, nearly sixty years ago, and am rereading Costain's books.
He bagan with a biography of his fellow Canadian, Alexander Graham Bell, and quickly found his feet writing historical novels. Like his other books, this shows the care he takes to study life in the era written about, in this case the Plantagenets (Henry II, Richard, John, Henry III, the emphasis being on how John's misrule resulted in Magna Carta). Unusually, the device used by Mr. Costain is science fiction, that is a careless angel allows a window in an American's mind to open to that time, allowing him to know of his life in these times. It is quite effective; I find I remember quite a few of the plot lines after all these years. I did forget the interaction of the main character with Stephen Langton, the Crusade, and Innocent III.
As is his custom, Costain includes romantic plot lines, in this case both at the turn of the 13th C. and in the 1950s.
I didn't bother to make an entry for this edition but posted it on the Pocket Books entry. This Avon Books edition of 1974 seems to be published to wring a few more sales, the Pocket Books paperback being from 1962 and the Doubleday first in 1957, when Costain was at the height of his popularity. As I recall, Avon republished a great many books and paperbacks were selling well into the 1970s, although the price per book rose, in this case being $1.50. As it was a bit different than most of Costain's back list (employment of the SF device) the cover shows a drawing taking up half the cover and including a ruin in the background, a horse carrying a knight, a lady, and a common man in the foreground. The artist had read the book! This is sold to readers by playing on the fact that his work is well known and so his name is in large letters at the top with 'master of the historical novel' in small type below it. The bottom of the cover has the title in type that is much smaller than that used for his name at the top. In very small type it says 'The fascinating story of a princess and a man who traveled through history.' It was only marketed as SF as an afterthought.
I find the story holds up very well.