I particularly enjoyed Bernard Cornwell's
Warlord series, starting with
The Winter King, which is another retelling of the King Arthur mythology.
The most similar author I've read is Morgan Llywellyn. My favorite book of hers is
Red Branch, about the Irish hero Cuchulainn. I seem to recall it had some light fantasy elements to it, but it does novelize some stories from the Ulster Cycle of Irish mythology, so...
Some of Guy Gavriel Kay's works have a similar
feel, though they do have fantasy elements and aren't based in "the real world" but rather fantasy analogues of real world historical times and places. For those books of his which are lightest on fantasy and heaviest on the historical feel, I would recommend:
- The Last Light of the Sun, based on the Viking invasions of Anglo-Saxon England and Wales.
- Under Heaven, based on 8th century Tang Dynasty (China) and the events leading up to the An Shi Rebellion.
- The Sarantine Mosaic duology, which starts with Sailing to Sarantium, based on Constantinople under Justinian I.
Of course, if you have no particular objection to fantasy I would heartily recommend anything he's written (except
Ysabel).