NEW YORK — The latest J.R.R. Tolkien project lasted six years, more than half as long as the author needed to complete his "Lord of the Rings" trilogy.

Getting permission to release a book in electronic form can be as hard, or harder, than writing it.

"The Tolkien estate wanted to be absolutely confident that e-books were not something ephemeral," says David Brawn, publishing operations director at HarperCollins UK, which announced last week that the late British author's work — among the world's most popular — would be available for downloads.

"We were finally able to convince the Tolkien estate that the e-book is a legitimate, widespread format."

Tolkien's addition to the e-club fills a major gap, and, with e-books the fastest (and virtually only) growing sector of publishing, other authors and their estates have softened. Former holdouts Tom Clancy and Danielle Steel have allowed their books to be digitized. Grove/Atlantic Inc., which has published William Burroughs and Malcolm X, expects many of its older works to become available.

"We're getting less resistance every day," says Grove associate publisher Eric Price.

But you could still build a brilliant collection with the books that remain off-line. They include, most notably, the "Harry Potter" series, and countless other favorites: "Catcher in the Rye" and "Catch-22"; "Lolita" and "To Kill a Mockingbird"; "Atlas Shrugged" and "Things Fall Apart"; "The Outsiders" and "Fahrenheit 451."

There are several reasons many books aren't available in electronic format: