Mystery at the Haunted Castle - Paul D. McDonald - Bog

Meet Pat and Patty Flaugherty (pronounced flair-tee), teenage fraternal twins who have a penchant for getting themselves involved in mysteries. They don't plan to become professional detectives, but never being ones to dismiss a challenge, they attack these mysteries wholeheartedly until they resolve them.
As a team, they each bring their unique strengths to the investigations: Patty's keen eye for observation of details and Pat's ability to "read" people. But with very different personalities, they are often at odds with each other during the process. Pat is headstrong and can be impulsive; Patty is levelheaded and practical, although sometimes too critical of her brother.
A business-pleasure trip to Paris sets off a series of incidents that involve the Flaugherty twins in their first mystery. A mysterious doll everyone seems to want, a legacy involving a cipher, and a ghostly knight haunting an old castle challenge Pat and Patty in their quest to help a newfound friend. As they are plunged into many dangerous adventures, including a fourth-story rescue operation, they uncover clues that lead them to discover the Mystery at the Haunted Castle.