How would approval work though, you can't expect apple to read your book to make sure there isn't offensive content.
Imagine someone uploads a childrens book that advocates drugs, murder, genocide or worse. Deleting it after complaints is not the way to go. Not worth the risk IMO.
They don't approve music when you submit it, why should they try to censor books? They should just mark anything not verified as "adults only". Let people parent their own kids and stop worrying about for them.