The Secret of the Minstrel's Guitar — Carolyn Keene

In this new mystery, the Dana girls' adventures take them to Portugal. Senor Sarmento, father of the girls' friend Isabel, has enlisted the help of the two young detectives in solving the series of baffling thefts from his Lisbon warehouse.

With great excitement, Louise and Jean set off for Portugal, but even before the ocean liner Balaska leaves New York harbor they find themselves involved in a mystery on board ship. What is behind the minstrel's magic music? Why is the handsome young man being pursued by a thief? Is the dangerous schemer somehow connected with the warehouse thefts? And what is the secret of the minstrel's guitar?