The Wall Street Journal includes a book index in its Friday edition, providing numerical ratings for books sold, where 100 is the average sales of the #1 book in the previous year. While I don't get the newspaper, I did visit a bookstore to check on the index score of a certain book released last Saturday.
Harry Potter and the Deathly Hallows had an index score of 1930.
And this index score is based on the sales of several major retailers (including Waldenbooks, Borders, Barnes and Noble, and Amazon). If it actually related total book sales, this figure would have been even higher, as sales at other outlets, including those that sell few books or almost no books, aren't included. Not many books are sold at the local grocery store!
And 1930 is an appropriate number. Come next year, looking at the stock price of Scholastic and Bloombury, you might think the year is 1930!