Trixie Belden #4 - The Mysterious Visitor - 1954
In this book, we get to meet a classmate of Trixie and the Bob Whites, Diana "Di" Lynch. Di started out as a regular girl next door, just like Trixie, but her family has recently become wealthy, and she is having trouble adjusting to changes in her life.
True to form, Trixie and Honey befriend Di and before you know it another mystery unfolds. Di's Uncle Monty (her mother's brother) has reappeared after years of living in Arizona. Suspicious Trixie thinks he's a big fake and can't help wondering if the Lynch family money has anything to do with his arrival.
The storyline in this book is similar to conflict between Jim and his uncle, but it is still a good mystery. We will be seeing more of Di and her family in future books.