It may be easier to say, first, what a book coach is not. A book coach is not a copy editor. A book coach is not a ghost writer. A book coach is generally not a line editor, although she may comment on a manuscript with line edits at some point in the process.
A book coach is a developmental, holistic editor, with the addition of supporting the writer through the entire process of writing a book, from start to finish, including time-management and publication guidance.
In other words, a book coach is a whole-book, whole-writer mentor.