Here be Dragons, by Sharon Kay Penman:
Sharon Kay Penman is often praised for how accurate her historical fiction is, but the real reason Here Be Dragons is so enjoyable is because of the characters. The minor characters have more depth than many author's heroes. But what of the main characters? There are three main characters, Llewelyn, the Welsh Prince, John, the English King, and Joanna, John's daughter whom he marries off to Llewelyn as part of a brief political alliance. Joanna is torn between them as the alliance falls apart, and her new position as Llewelyn's wife illuminates the flaws of the father she had previously adored unquestioningly. Much of the story is how Joanna comes to terms with her feelings toward John, and how this impacts her relationship Llewelyn. All three are masterpices, and John is perhaps the most intriguing of them all. He is ultimately unforgiveable, but, at the same time the reader finds him to be equally un-hateable.