I removed the book delete feature because I want to encourage people to delete only the content they really don't want anymore.
Since Bookvoid lets you create mashups where books can contain parts of other books, deleting an entire book risks making holes in other author's books. You can always empty all the sections and subsections of your book. It's not a bad idea to replace deleted content with a message explaining to readers why the content has disappeared and maybe linking to where they can find it if it's still somewhere.
How about at least a setup where authors have the first 24 hours from creation to easily delete their book, that will give lookie-loos like me a chance to cleanup after toying with the site.
