The First Law trilogy by Joe Abercrombie. No promises on the characters, most of them are not so rational, but you'll see why I said it by the end. There are more books in the same setting with some of the same characters if you like them. The first book is mostly setup but it is great after that.
This is a thread where people can ask questions that they would ordinarily feel embarrassed for not knowing the answer to. The previous thread is at close to 500 comments.