The overview is not quite a synopsis, it’s not quite a sample chapter, it’s not quite a catalogue copy, it’s not even quite, an overview. It is in reality the representation of the book you’re going to write. It’s a sales pitch – the story/ narrative / subject in brief – what the book is going to be about, what it will be like and who’s going to read it.
So to write the overview, you’re telling the gist of the story. A good overview will give the agent/editor a great sense of the subject, the scope, the heart, and the need for the book. It needs to capture hearts and get people enthused about the project. The focus here is on summarizing and pitching the project, making it sound appealing and yet extremely necessary.