The Name of this Book Is Secret (The Secret Series)

This is the story of a secret, and also a secret story. Cass and Max-Ernest, two friendless 11-year olds find a mysterious case containing vials of liquid labeled The Symphony of Smells with an encoded message asking for help, they decide to investigate. Along the way they must solve puzzles and word games and face incredible danger to answer their questions. They encounter the dark and looming Dr. L and his seemingly ageless accomplice, Ms. Mauvais, who have kidnapped one of their classmates in a desperate attempt to unlock the secrets of immortality.