Shapestone by James Bibby
Death is nasty and not a little inconvenient. Being dead even more so. Especially when you're tied to an amulet that has its own evil designs on the world. For the Princess Macoby the annoying ghost with the axe in his head who keeps hanging around trying to remember swear words is just plain irritating. Little does she know that he holds the key to the mystery of who killed her intended and betrayed her father's army and the secret of the ancient evil that is aiming to overwhelm the land. (All the usual stuff). All she wants is to find the truth. All he wants is to be dead in peace. It's the Shapestone that is standing in the way.