Now I'm really going to have to reread it to see if I spot the connections! Because I don't remember thinking that the Thief books were All About Loyalty, and normally I'm all over stories that strike me that way. I haven't read the most recent book (books? I'm not actually sure which was the last one I read) in the series, so that might be a good prompt.

I remember I spent a great deal of time combing my school library for books that might have the same hit of Intense Loyalty Feels that Sutcliff did. There were some other historical novels that went in similar directions - I still have great fondness for the absurdly titled One Is One by Barbara Leonie Picard, which is all about a young squire and his knight, and To Spare the Conquered by Stephanie Plowman, which is about a young Roman officer during Boudicca's revolt in Britain and his various loyalties - but nothing ever could quite match the reliability of Sutcliff.