Answer:
Taking out the 25% of Americans that didn't read a book last year {washington post article), the average is ~15 books a year (per reader). Doing a little bit of math (putting in the zeros and doing an average) we end up with about 9.
http://www.washingtonpost.com/wp-dyn/content/article/2007/08/21/AR2007082101045_pf.html
http://writtennerd.blogspot.com/2007/08/link-mad-response-american-reading.html