One of the classic Boxcar Children books. The mystery revolves around a divisive family well-known in a coastal New England town, and the aftermath of the mystery focuses on how the four children and their grandfather can help them.
My daughter has just gotten interested in the Boxcar Children books and now wants to collect the whole series. She hasn't been much into reading chapter books up until this point, but it seems to be a good jump from picture books to chapter books. There are short chapters, pictures scattered throughout and a mystery that keeps things interesting. Highly recommended for first time chapter readers!