It had to happen: Amazon.com announced Thursday it is selling more Kindle ebooks than print books, either hardcover or paperback.

Amazon said since 1 April, it has been selling 105 Kindle ebooks for every 100 print books. Free Kindle books are excluded from that count and if free books were included, the number would be even higher.

Amazon CEO Jeff Bezos said the ebook threshold arrived sooner than expected. "Customers are now choosing Kindle books more often than print books," he said. "We had high hopes that this would happen eventually, but we never imagined it would happen this quickly." Amazon has sold print books for 15 years and Kindle books for less than four.

Amazon doesn't share sales numbers, but said it had sold more than three times as many Kindles books thus far in 2011, as it had in the same period in 2010.

The ebook surge is one of the biggest indicators of the impact of technology on culture, and the nation's public librarians have been careful observers of the trend for years. Many public libraries already offer ebook borrowing for free, subject to a library user loading special software on a desktop computer, mobile device or some ebook readers.

Amazon's news came as a surprise to Deborah Ervin, head of reference at the Framingham Public Library, a mid-sized library located in a Boston suburb.

"I'm certainly not opposed to [e-readers]," she said in an interview. "They are something people like and enjoy." Ervin enjoys using the Nook e-reader, made by Barnes & Noble, which works with OverDrive software offered by her library and others in the Boston area's Minuteman library chain for free ebook borrowing.

The convenience of replacing a heavy printed book, or several, on a lightweight e-reader is appealing, especially for somebody taking a long vacation, Ervin said.

"This is the latest and greatest technology, although many don't embrace it for one reason or another and might prefer holding an actual book or feel e-readers are too complicated," she said. "Some people buck modern trends and would use a rotary phone if they could. Still, a lot of people use ebooks almost exclusively."

While the Nook and Sony e-readers allow open ebook library borrowing, Ervin said she and other librarians are hopeful that Kindles will soon expand beyond borrowing only Kindle books from libraries. (In April, Amazon announced it would allow Kindle book-borrowing from 11,000 US libraries later this year. )

Ervin said users of e-readers tend to favour ebook purchasing rather than borrowing, since the software and download is relatively quick and easy for a purchase.

"Borrowing an ebook is overwhelming sometimes, and the way OverDrive works is complex for some people to get onto their devices, whatever those might be," she said. First, you have to see what book is available and then download the e-reader software and then load the actual ebook onto the device, she said. "It's cumbersome".

Ervin said she's not really misty-eyed for printed books and believes that soon enough ebook borrowing will be much easier and more commonplace. For now, the inevitable replacement of print books with ebooks continues unabated.

"In my own reference department, we have in the past ordered several titles of literary criticism in paper, but this year we stopped those and have them as ebooks," she said. "That way, anybody can read them, even when the library is closed."