The Story of the Voyages.

In this fantasy with dragons and flying machines, 10-year-old Marta is determined to find her brother who was kidnapped at Christmastime by a fake Saint Nick because the little boy is blessed with the ability to locate gold. A kindly, wise philosopher-type by the name of Orlando joins Marta in her search, and together they survive a gigantic sleeping dragon and imprisonment in a tower with no clear means of escape. They overcome that hurdle, but later on, Orlando succumbs to the evils of The Plague (an all-consuming woman!) and Marta continues on her search alone. Eventually, she does find her brother -- but that only introduces problems she had never considered.