I guess they will have completely arrived when all in-print books are available electronically, and it becomes unthinkable to publish a book without a digital edition. Rights-holders have digitized most of the still-selling back catalog of books. Most books you are interested in, new or old, are available digitally some how.
E-readers (or general purpose devices that can read ebooks) are cheap, maybe about the cost of a mp3 player/the cost of a portable CD player. Most people have one, even if they rarely read books.
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