Things Hoped For (by Andrew Clements)

Then she meets Robert, a trumpet player also in town for auditions, who has a secret of his own. Having shared their individual secrets, together they discover one more in Gwen's basement, a secret that shatters Gwen's quiet, regular life and her ability to concentrate on auditions, yet deepens their friendship in surprising ways.

This is a sequel only in that the main character from the first book enters as a supporting player partway through, bringing along the invisibility theme, which is the least interesting plot element here. Of more interest are the details about being a gifted young musician and living in New York. But this is primarily a character- and idea-driven piece, and even when some of the plot elements don't quite make sense, the ideas are intriguingly open-ended, and the characters are warm, real, and moving.