add a librarian note
The "add a librarian note" link is located above the Title field on the book edit page. Clicking it will bring up a text box that will allow you to attach a note regarding important information about the book. An important note may be about an ISBN conflict, that the edition is an alternate cover edition, that the book should not be combined with certain books, or anything else pertinent to cataloguing the book in the Goodreads database.
Librarian notes of this type are edition specific and will show up in a yellow box near the top of the book edit page and also in a yellow box below the specific edition on the author's combine page.
If a book has a Librarian Note attached when you go to edit it, please read it carefully and keep it in mind when doing any editing or combining. If a note has you confused, you can always post to the Goodreads Librarians Group and ask for help.
Here is an example of a book with a Librarian Note: LINK