Possible Clues:

• Addendum - vestigial pouch in human intestines
• It's useless in the body, but useful in a book
• Leaves having no function inside trunk?
• Add nine and have it out if necessary (8)
• Add eleven back at the end of the book
• Final part of work often discarded in theatre
• Outgrowth of intestine, often surgically removed
• Supplementary information at end of book
• Book back part
• Add nine extra pages