I think Robert Louis Stevenson is one of the best historical writers. Kidnapped was intense, and the characters are just downright awesome. I also recommend Robert E. Howard (who was a genius in my book). Most of his Conan stories are just as harsh and gritty as A Song of Ice and Fire. But if you really want to get real gritty, read Jack London (whom Howard loved). That guy knew how to write raw and mean, especially in The Sea Wolf (the best of his works).