Please add suggestions!
  1. Shakespeare's autobiography
  2. The manuscript(s) Ernest Hemingway left in a suitcase at the train station
  3. A book by Ambrose Bierce about everything that happened to him after he disappeared
  4. Lost plays of Sophocles
  5. A book revealing all of the US government secrets
  6. The next novel John Kennedy Toole would have written
  7. A book by an adult Anne Frank
  8. More epic poetry by Homer
  9. The Sylvia Plath journals that Ted Hughes destroyed
  10. A fully realized second novel by Harper Lee that has nothing to do with To Kill a Mockingbird
  11. Another Lord of the Rings installment
  12. A book by Kilgore Trout
  13. The actual A Hitchhiker's Guide to the Galaxy
  14. Handbook for the Recently Deceased (from Tim Burton's BEETLE JUICE)
  15. A book by Dante not about Heaven, Hell or Purgatory but about his everyday life on Earth.
  16. More by Bruno Schulz
  17. The finished and complete Mystery of Edwin Drood by Charles Dickens
    Suggested by @eatthelove