This is one of my favorite books of all time by one of my favorite authors of all time. Sharon Kay Penman is a master at presenting the shades of gray within a conflict. She never falls back on "This is the villain. He does this because he is the bad guy" or the reverse for the hero. Everyone in her stories is so believable and realistic and you truly understand why things turned out the way they did. Plus, her historical facts are great.
This story is about Llewelyn, one of the last "independent" powers in Wales and his struggle to retain independence from the English empire. To that end, he marries King John's (almost always portrayed as wicked with no redeeming qualities but here made very human and believable, even if not very likable) bastard daughter. This story follows their marriage and the problems of medieval kings and powers. This would be one of the books I would have to have on a deserted island.