Someone who was once briefly Rosemary Sutcliff’s editor (I do not know where or when) used to post as Antonius Pectinarius at www.ancientworlds.net . He believed her best work was in the 1950s and 1960s, beginning with The Eagle of the Ninth and ending with The Mark of the Horse Lord which was his own favourite. Writing in 2003, he said:
She had, as did Henry Treece, a mystical communion with the past, which enabled her both to recreate tiny details, and to confound military historians with her understanding of the art of battle in any situation she cared to devise. Her sense of place was uncanny, in that she could get no nearer to a site than the seat of a car on an adjacent road. Friends often served as her eyes, and also as her researchers, but it was the conclusions she drew from the evidence, and her re-creations of them, that made her contribution to the literature about the ancient world so distinctive.
Where she was simply embellishing recorded history, she was no better than anyone else.
She also had one of the rudest senses of humour in anyone I have met.
Source: Rosemary Sutcliff—more appreciation.