The Mystery of the 99 Steps
by Mildred Augustine Wirt Benson
"Despite warnings from a stranger, Nancy, Bess, and George travel to France to search for a flight of 99 steps to solve the mystery of a friend's strange dream. During her search, she must contend with the sinister Monsieur Neuf and help her father uncover who is frightening a wealthy financier into selling large amounts of securities."
