Amazon has released its fourth quarter financial statement marking two significant milestones: the latest period was Amazon's first $10 billion quarter with net sales increasing 36% on-year to $12.95 billion, and the company's Kindle ebook sales have surpassed paperbacks.

Last July, Amazon announced that it was selling nearly two ebooks for every one hardcover. The company originally thought ebooks would outpace paperbacks by the second quarter of this year, so the milestone came unexpectedly fast (and it's on top of increasing paperback sales).

Amazon now sells 115 Kindle books for every 100 paperbacks. Meanwhile, the gap between ebooks and hardcovers has increased to three digital sales for every physical copy. This includes traditional books that don't have a Kindle copy and excludes pre-1923 public domain works.

The US Kindle store now offers more than 810,000 books, up from 630,000 last summer, as well as millions of free titles with expired copyrights. Amazon has yet to share device sales, but the company touts that "millions" of the third-gen Kindle were sold in the fourth quarter of 2010.