Looking for a historical fiction book series

Hi, im looking for a historical fiction series i read a few years ago that follows this american librarian who has a book shop in Copenhagen but who frequently travels around the world to investigate historical mysteries. From what I remember, some of the books follow crusaders who moved the holy grail to a safe spot, recovering lost art stolen by the nazi, recovering the original prussian room made of amber. Ill try to update if i remember any more details lol but thats all i remember rn and i cant seem to find it