Monday was a day for the history books  if those will even exist in the future.

Amazon.com, one of the nation’s largest booksellers, announced Monday that for the last three months, sales of books for its e-reader, the Kindle, outnumbered sales of hardcover books.

In that time, Amazon said, it sold 143 Kindle books for every 100 hardcover books, including hardcovers for which there is no Kindle edition.

The pace of change is quickening, too, Amazon said. In the last four weeks sales rose to 180 digital books for every 100 hardcover copies. Amazon has 630,000 Kindle books, a small fraction of the millions of books sold on the site.

Book lovers mourning the demise of hardcover books with their heft and their musty smell need a reality check, said Mike Shatzkin, founder and chief executive of the Idea Logical Company, which advises book publishers on digital change. “This was a day that was going to come, a day that had to come,” he said. He predicts that within a decade, fewer than 25 percent of all books sold will be print versions.