The Name of this Book is Secret - Pseudonymous Bosch, Usborne Publishing

When intrepid friends Cass and Max-Ernest set out to discover the history behind the mysterious Symphony of Smells, they become embroiled in an experiment to discover the secret of eternal youth. Together they must come up with a plan to rescue their classmate from a hideous fate and escape the eerie pyramid of dastardly villains The Midnight Sun.