Electronic books may be the fastest growing segment of the publishing world, but some authors are still not interested in participating. You won't, for instance, find any of the Harry Potter novels in digital format.

Getting permission from an author – or an author's estate – to release a book in electronic form can be as hard, or harder, than writing it, explains Hillel Italie in a piece for the Associated Press.

There are various reasons why some well known works – including the Harry Potter series, "Catcher in the Rye," "Catch-22," "Lolita," "To Kill a Mockingbird," "Atlas Shrugged," "Things Fall Apart," "The Outsiders,"  and "Fahrenheit 451" – have not been adapted to the e-book format, writes Italie.

Sometimes it's because the author (like J.K. Rowling) prefers paper books. Other times it has to do with a dispute over money and the suspicion that publishers are making too much profit from the electronic format. Sometimes it's concern that the market for e-books is not yet big enough to justify the expense and trouble of digitization.

But there are other occasions, says Italie, when the author is the last to know. Erica Jong, for instance, told Italie that she "had no problem with e-books" and was surprised to learn that her 1973 novel "Fear of Flying" wasn't available in that format.