Traditional bricks & mortar book stores have been losing ground in terms of market share of all book sales in the United States for a decade now. While the total volume of books sold is on the rise, those sold by companies that run bricks & mortar book stores have declined from 61.5% in 1997 to 50% in 2007. The rise of Amazon to prominence in the book selling market is, of course, a significant reason for this shift. Traditional booksellers are working hard to recapture their prominence in the retailing of books by establishing complex networks for selling electronically and selling eBooks.

Geographic reference: United States
Year: 1997 and 2007
Market size: $9.71 billion and $12.33 billion respectively
Source: 1997 Economic Census: Sector 44: Retail Trade: Merchandise Line Sales: Merchandise Lines by Kind of Business and 2007 Economic Census: Sector 44: Retail Trade: Merchandise Line Sales: Merchandise Lines by Kind of Business for the United States: 2007, October 30, 2009. The 1997 data is available online here and the 2007 data is available online here. The September 3, 2010 Dwarf Planet Press blog post presents these Census Bureau data in a summarized fashion. Here’s a link to that post.
Original source: U.S. Bureau of the Census