This one was a doozy, but a good doozy. The Winter King is the first book in a trilogy that Bernard Cornwell calls "The Warlord Chronicles." It's a retelling of the King Arthur legend, but with extra attention paid to incorporating the scraps of historical data we have from back then, as well as a dark, gritty, and earthy interpretation that refuses to whitewash the characters or the setting.
It was a really, really good book.