I have only just realised a subtle titling difference.
The Rider of the White Horse (UK edition) by Rosemary Sutcliff
From a 5/5 Amazon reader’s review:
My sister and I agreed on few things when we were young, but this book was one of them. We both loved it and still do. I have a copy purchased long ago. This book about Thomas Fairfax – an English Civil War General on the side of Parliament – told from the viewpoint of his gallant wife – is magnificent. I treasure it. It moves me and I believe in the authors portrayal of Anne and Thomas Fairfax. They live and breathe within this book – as indeed do all the characters. Neither Anne nor Thomas thought that the King should be executed and Thomas refused to agree to it although the book does not reach that point in history – but their characters are so well portrayed that you can well believe it.
And here some more covers of UK versions:
ROSEMARY SUTCLIFF (1920-92) 
I’ve always been curious about why the title was different for some editions of this novel. RS herself wrote it on her list of adult books as “Rider of the White Horse”. Maybe the answer is that it was altered to “Rider on the White Horse” for the US market? Different US/UK titles are quite common and cause all sorts of confusion for buyers. “Rider on a White Horse” might have been seen as being more familiar from “Revelations”‘. My copy was titled “Rider on a White Horse” and I had wondered if RS took the title from the biblical quote: “I saw heaven standing open and there before me was a white horse, whose rider is called Faithful and True. With justice he judges and makes war..” Seemed to fit her understanding of Fairfax so perfectly :)
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My copy has the middle cover of the group of three, the mostly-white one. It is so hard to believe that this book was written before I was born!
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