She's most likely already read them, but C.S Forester's novels are sort of the pinnacle (Horatio Hornblower; The African Queen). Bernard Cornwell is also pretty good (Sharpe). My personal favourite, though they are more fiction than history, are Giles Kristian's books (he did a Viking trilogy and is now working on the English Civil War). The Three Muskateers is historical fiction, but again she's probably read that. The Cadfael books are excellent.
There's loads to choose from, but all the ones that come to mind for me are the huge ones, sorry.