This is an addendum to to a previous question: How is the copyright of the derivative work dated? For example; an author writes a book in 2009 and in 2020, re-edits the book with new material. Does this new material benefit from the earlier copyright? What if the author is suspected of infringement with this new material only, in 2021, by a second author who published his work in 2015? Is the first author accountable to the second? How can it be proven if all the old copies have been removed from the shelves?
The author of a derivative work only gets a copyright on content that is original to the derivative work. There is no new copyright on content from the source work. The author of a revised edition only gets a new copyright on new content in the revised edition (provided that it is original).
This used to be more important when a copyright term was calculated from the year of publication. Now it is important only in edge cases, such as when it is claimed that the new content was copied from another work, and is infringing. The date of publication is now less important because the copyright in most things that a specific author writes expires at the same time: 70 years after he or she dies (works published anonymously and works made for hire are exceptions to this rule).
The relevant US law is 17 USC 103, which provides that:
(a) The subject matter of copyright as specified by section 102 includes compilations and derivative works, but protection for a work employing preexisting material in which copyright subsists does not extend to any part of the work in which such material has been used unlawfully.
(b) The copyright in a compilation or derivative work extends only to the material contributed by the author of such work, as distinguished from the preexisting material employed in the work, and does not imply any exclusive right in the preexisting material. The copyright in such work is independent of, and does not affect or enlarge the scope, duration, ownership, or subsistence of, any copyright protection in the preexisting material.
If Author A writes and has published a book in 2009, A owns the copyright on it (unless A sells or transfers it). Let us suppose that Author B writes a different book on a related subject in 2015, and then A publishes a revised edition of the 2009 book in 2020. In 2021 B accuses A of having copied content from the 2015 book without permission, and incorporating such infringing content into the 2020 edition. A claims that this content was already present in the 2009 edition. How can A prove this defense?
Commercial publishers pretty much always retain a few copies of every edition, even if a newer edition is the only one offered for sale. In any case, a commercially published US book will essentially always be registered with the Copyright office, whch means that a copy will have been sent to the Library of Congress, and that copy can be checked.
Moreover, B is not likely to even file such a suit unless B's lawyers have a copy of the 2009 edition and can show that the copied content was not found in it.
If it were proved to the satisfaction of a court that A had, in the 2020 edition, copied from B's 2015 book, A would be liable for damages to B, the amount determined by the court, and B could also get an injunction stopping distribution of the 2020 edition.
Laws in other countries on these points are generally similar.