(USA TODAY) -- David Delk, 47, remembers "the old days," when his extended family went on vacation carrying three or four bags filled with about 30 books and another 30 magazines.
In a digital age, packing is easier.
Last summer, Delk; his wife, Ruthie; their three children, ages 17 to 22; and nine other relatives went on their annual family vacation equipped with a variety of reading devices.
"There were probably seven Kindles, nine iPads and two other tablets between the 14 of us," says Delk, president of Man in the Mirror, a Christian ministry in Casselberry, Fla.
And while Delk still buys physical books - "when you really want a copy of something solid and stable'' - most of his reading is done on his Kindle or iPad.
He's in the minority, but it's a growing minority.
A poll conducted for USA TODAY and Bookish, a website designed to help people find and buy books, finds that 40% of adults - including 46% of those ages 18 to 39 - own an e-reader or a tablet. That's more than double the numbers less than two years ago.
Reading devices are even more popular among college graduates (60% say they have one) and those with annual household incomes of at least $75,000 (62%).
According to the poll, 35% of those with reading devices say they're reading more books since they got their reading devices.
Delk, who travels frequently for work, says he's up to about 40 books a year from 25.
In the past few months, he's raced through all 14 books in Jim Butcher's fantasy/mystery series, The Dresden Files, mostly downloaded from the Orange County (Fla.) Public Library. "As soon as I finish one, I download the next."
He notes, however, that his son, Kyle, 17, a high school senior, prefers turning physical pages to reading on screens because it's easier to focus and know where he is in a book. Most importantly, Kyle says, "I like the way the book feels in my hands."
But with a growing number of digital readers, "the key breakthrough in publishing is accessibility," says Peter Osnos, founder and editor at large of PublicAffairs Books.
"For hundreds of years, when readers heard about a book they wanted to read, they had to go find it," he says. "In an digital era, everything, or most everything, is instantly available."
That appeals to readers such as Jamie Groves, 45, a prosecutor in Huntington, Ind., who reads about 30 books a year - double his reading before buying a Kindle five years ago and adding an iPad three years ago.
A fan of novelists such as Brandon Sanderson and Stephen King, Groves likes both the cheaper prices of e-books and "having something to read even if I forget to bring a book. I like to read to fill in the gaps, like waiting in the dentist's office or having lunch."
That e-reader owners say they're reading more books doesn't surprise Michael Norris, a publishing analyst for Simba Information, a market research firm.
He compares that to saying "a person who owns a bicycle is more likely to take bike rides. If you have a device, you have access to books that you just didn't have before, and a lot of them are free."
The poll asked a variety of other questions:
• Why do you read books?
To learn something (72%), to be entertained (64%), to be able to talk with others about the books you've read (19%).
• What keeps you from reading more books?
More than half - 51% - cite lack of time as a major factor. Only 16% say lack of interest in reading; 14% cite a lack of quality books.
• How often does a book play a role for you in meeting a new friend or romantic partner?
Never, say 78%. But others says romances and friendships do spring from books often (3%) or sometimes (7%).
• And for those reading more because of their devices, what kinds of books are you reading more of? (Readers could list up to three genres.)
Nearly one in four - 23% - mentioned science fiction or fantasy, followed by mystery and crime (16%), romance (14%) and non-fiction (14%).
Osnos says that while the accessibility of books has been transformed, there's the question of "visibility: how people discover the books they may want to read. The traditional place to do that was bookstores. You'd go in to buy one book and discover another."
Officials at Amazon and book websites contend that clicking can replace browsing; traditional booksellers argue that's not the same as personal recommendations or what they call "handselling."
The poll also asked readers what factors create interest in a particular book for them.
A majority (57%) cite their own opinion of the writer's previous work as the major factor. Opinions of a relative and friend (publishers call that "word of mouth") came in second at 43%.
Lower on the list of major factors: professional reviewers and other writers (each 17%), the book cover (16%) and Internet opinions by non-professionals (10%).
When asked if they share their opinions of books online, only 27% say they've used Facebook, Twitter or book websites to comment on a book. But among those under 40 who own a reading device, 50% have posted online comments.
That's where Nancy White, 29, an office manager in Chicago, often hears about books, especially the kind of historical fiction she likes. "But nothing beats hearing about a book from someone you know and whose taste you trust," she says. "The Internet stuff helps, but I still like talking to my friends about books they like."