Dear Americans who like books on paper, there's
a new trade paperback edition of The Mark of the Horse Lord projected this July from Chicago Review Press (the people responsible for the current, green-covered edition of Sword at Sunset) in their "rediscovered classics" line.* Yes, there is already an e-book edition available from Red Fox.
*Also includes an amusing-looking 1950s Boudicca vs. the Ninth Legion novel by Anya Seton,
The Mistletoe and the Sword, and a whack of other historical novels.
If you haven't read it, The Mark of the Horse Lord is one of Sutcliff's best and most dramatic novels, despite the premise sounding like The Prince and the Pauper: Roman Britain (With Gladiators!)
If you have read it, consider starting an edit war on the
TV Tropes page I made and then callously abandoned. It could use some crosswicking.