ALAN ARMSTRONG liked to read books such as "Captains Courageous, Kim, " and "Treasure Island." As an adult he has worked as a lawyer, a traveling bookseller, and an author of books about children who are put to the test. "Moon Girl" is his fourth book for children. His first, "Whittington, " was awarded a Newberry Honor in 2006. He live in Massachusetts with his wife, Martha, a painter.