Mystery at the Spanish Castle by Ruth Nulton Moore, Ruth N. Moore
When Sara sees a strange light in a villa on the island across the bay, she is surprised and curious. Nobody has lived in that Spanish castle for a long time. Then what is the wavering glimmer doing in its tower room late at night? At first nobody believes Sara saw the light, not even her twin, Sam. Their friends at Bayport think it would be fun to explore the castle. When they hear a weird cry in the tower room and see the ghost on the stairs, they know something bizarre is going on. Then, a boy arrives on the scene and tells them that he and his mother are now living in the castle. Sara and Sam know they first have to win his trust in order to solve the mystery. By taking the first step in being friendly, they help their new neighbor out of his troubles. At the same time they unravel the mystery at the Spanish castle.