If there are no subpages in a book (just a root page), the book admin page admin/content/book/%nid shows a table header with no content in table. According to http://drupal.org/node/604342 and empty table should contain an explanation and/or call to action.
Proposed wording if there is no content in the table:
No book content available.
Why not call to action link? Concistency: we currently do not do it for content admin page. Create a separate issue to contest / sync this.