Here’s where it gets complicated.
Not so long ago, book publishers had no idea that there would be such things as digital books. Publishing contracts mentioned nothing about ebooks. Literary agents made sure to keep derivative rights separate, so that translation rights, film rights, stage adaptation rights, etc. for a successful book could be separately monetized.
When ebooks started to become important, the digital publisher Rosetta Books took advantage of the situation, and started acquiring ebook rights to well-known books such as Kurt Vonnegut’s Slaughterhouse Five. This did not please the print publishers at all. Random House, one of the “Big Six” US publishers, took Rosetta to court, saying that their publishing contracts gave them exclusive rights to distribute books, and ebooks were books.The coffee’s on, let’s get to work.