SITTING at a table in a Barnes & Noble in St. Petersburg, Fla., T. J. Waters was signing copies of his book “Hyperformance” when a fan standing in line with the eBook version walked up and said, “It’s too bad you can’t sign my Kindle.”

Mr. Waters, a senior consultant for United States Special Operations Command Headquarters, suddenly found himself wondering, “How come the tech world can put a man on the moon and I can’t sign an eBook?”

With consumers now accustomed to experiencing music and movies on Lilliputian devices, they’re also increasingly reading Tina Fey, Scott Turow and Suzanne Collins on Kindles, Nooks and iPads. You can be a die-hard Haruki Murakami fan and not have a single novel of his on your bookshelf.

And that’s only going to become more common. By 2015, sales of eBooks in the United States are expected to triple to nearly $3 billion, according to Forrester Research. But the sea change has created a dilemma: what, then, do authors autograph at book signings?