When Passion Rules by Johanna Lindsey

When Passion Rules - Leonard Kastner is an accomplished assassin.  His last client has hired him to kill the infant princess of Lubania.  Instead, Kastner kidnaps the baby and flees to London.  On her 18th birthday, Alana Farmer learns that she is the princess and needs to return to her father to help avoid a civil war.  Chrisoph Becker is the captain of the guards.  He arrests Alana as an inposter and interrogates her to learn who is behind her attempt to infiltrate the royal family.