Web-only retailers in the United States accounted for 43.8% of book purchases by volume throughout most of 2012, up from 35.1% in 2011, according to Bowker Market Research. The projections for both years cover the period from January through November.

The report says large bookstore chains accounted for 18.7% of U.S. book sales from January through November last year, down from 28.7% during the same period in 2011, and 31.5% in 2010. The rest of the sales came via such venues as independent bookstores—3.7% during the 2012 and 2011 periods, up from 2.4% in 2010—warehouse stores, mass merchandisers, supermarkets and book clubs.

Part of the decline stems from Borders, the second-largest bookstore chain after Barnes & Noble Inc., going bankrupt and subsequently closing its 399 stores in the summer of 2011. Based on its 2010 sales, Borders had been No. 194 in the the Internet Retailer Top 500 Guide. BarnesandNoble.com is currently No. 32 in the Top 500.

In November 2012—the latest data available from the market research firm—about 28% of all book purchases in the United States were of e-books. That compares with approximately 19% in November 2011. “It is clear that the e-book format has really come of age in the U.S.,” says Jo Henry, director of Bowker Market Research. “E-books’ market share has seen steady growth since January 2009, with steep rises after each Christmas.”

According to the most recent edition of the Internet Retailer Top 500 Guide—soon to be supplanted by the 10^th anniversary edition—the books, music and video product category, as measured by sales, increased 20.4% year over year in 2011. Consumers that year spent more than $6.2 billion from Top 500 merchants included in that category.

The Bowker report also measures online book sales in the United Kingdom during the same time periods as the U.S. estimates. By volume, web-only retailers accounted for 37.7% of U.K. book sales in 2012, up from 30.4% in 2011. Chain bookstores accounted for 27.6% in 2012, up slightly from 26.7% in 2011 but down from 30.0% in 2010.

In November 2012, 9.3% of U.K. book purchases were of e-books, down from 13.0% in July—that summer month being, the report says, the “height of the Fifty Shades of Grey phenomenon,” referring to the erotic narrative series.