I finished reading this for the 4th time this morning and the story still keeps me up late at night, turning pages. I've read probably 25 books by Bernard Cornwell, mostly from the Sharpe Series, and the Warlord Trilogy (of which The Winter King is the first installment) is far and away his best work.
Cornwell's style is one of assiduous historical accuracy, and the Sharpe books are worth reading just for their portrait of life in the early 19th century. The difference in his Arthur books is that there is so little written record of the time (~500 AD). Cornwell dilligently researches these books as usual, but in the end there are much wider gaps which need to be filled by the storyteller. This forces him to put his story first and history second.
Happily, the author agrees with me.