Where can I find detailed book sales figures or statistics?
Strange as it may seem, we know of no reliable, publicly-available way to get comprehensive statistics for book sales at this time. The only database with reasonably accurate information is Nielsen BookScan, which reports point-of-sale data, but even that claims to represent only 75% of all retail sales. BookScan is a recent (last ten years), very expensive subscription service, used primarily in the industry. Harvard does not have a subscription.
If you are looking for current sales figures, you can use Amazon.com to get a general idea, although it relies on very recent numbers, so the figures are not really accurate. The “Author Central” feature on Amazon.com does provide BookScan data to current authors only.
Prior to BookScan, the only source for market data for some individual titles was Best Seller lists compiled by publications such as the New York Times that survey selections of book stores from which they generate estimates of rankings. However, these published lists don’t indicate how many copies of a book have sold or the relative sales among books on the lists. Since 2009, BookScan does supply weekly “book charts” to the Wall Street Journal, but they are just best seller lists. The bottom line is, only book publishers have comprehensive sales data, and they don’t usually make it public.
If you are looking for historical statistics for particular books, publishers' archives are the only reliable source. A book called A Guide to book publishers' archives, available in the Loker Reading Room in Widener (call number RR 101.18), identifies archival collections.