Yes, it's finally happened. The digital book has overtaken the paper book on Amazon.com. And this time that isn't only true about hardcover books. Now we're talking paperbacks.

In reporting its latest earnings, Amazon said that it was selling more Kindle books than paperback books, though the score is still close. Since January 1, for every 100 paperback books Amazon sold, 115 Kindle books were sold. To top it off, the company says that since the beginning of the year it's sold three times as many Kindle books as hardcover books. Amazon noted that this data was from "across Amazon.com's entire U.S. book business and includes sales of books where there is no Kindle edition." It added that free Kindle books were excluded from the tally.

Yes, indeed, the sands are shifting.

While Amazon has said previously that Kindle e-book sales were outpacing hardcover sales, this is the first time it's said that Kindle books were outselling paperbacks, which typically cost significantly less than hardcovers. As usual, Amazon didn't report exact sales and would only say that it had sold "millions of third-generation Kindle devices."

Some reports suggest Amazon sold more than 8 million Kindles in 2010. Combine that with all the people buying the iPad, iPhone, and iPod Touch, as well as millions of Android-powered devices, and you can see how Amazon's digital book sales got a huge boost. (Amazon, like Barnes & Noble, offers its e-reading Kindle app across multiple platforms).

As for total numbers, all Amazon would say was that it currently has 810,000 books in the Kindle Store and that doesn't include the millions of free, out-of-copyright, pre-1923 books that are also available as e-books.

What's this mean Well, it's not good news for brick-and-mortar bookstores. At the recently held Digital Book World conference, James McQuivey of Forrester Research presented some research findings before a CEO panel on Tuesday. He included the estimate that consumers spent about $1 billion on e-books in 2010 and that sales should reach at least $1.3 billion in 2011. McQuivey said that the consensus among those surveyed was that e-books would constitute half of all trade book units by 2014, and 53 percent said they expected print sales to decrease this year.

At least one guy, Mike Shatzkin, a conference organizer and head of the Idea Logical Company, said that within 10 to 12 years brick-and-mortar bookstores would "more or less disappear." However, some believe that the "downsizing" of brick-and-mortar superstores might actually bode well for independent bookstores, which in some ways are better equipped to tackle what may indeed become more of a niche business in the years to come.