The Winter is Coming website has a post up, What books should you read as you wait for The Winds of Winter? (The Winds of Winter is the next book).
I don’t have much time for fiction at this point, but the first entry that they suggested was . This is a very dark, gritty, and realistic, retelling of the Arthurian legend, written in a fashion more reminiscent of historical fiction than fantasy. I read this series perhaps a year after first reading , and was struck by similarities of tone.
As it happened this was before George R. R. Martin was quite as famous, and I emailed him at some point in 2000 about various issues relating to his works and inspirations, and asked him about Cornwell’s series. Martin admitted that he was a huge fan, and appreciated that there were similarities of style and tone.
In any case, I second this recommendation. is not the most easy read…but worth it.