Sherlock Files, Book 1: The 100-Year Old Secret

Xena and Xander Holmes are the great-great-great grandchildren of Sherlock Holmes, and they have inherited his powers of deduction. Invited to join the SPFD (Society for the Preservation of Famous Detectives), they are gifted with a musty old leather-bound book titled: SH: Unresolved Cases. These are the cases their ancestor never solved. One case intrigues them - The Girl in the Purple Hat - and they find themselves on the trail of the clues scribbled in the old book.