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 comparatively recent (2005), 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 anything like market data for some individual titles was Best Seller lists compiled by publications such as the New York Times. These are not based on point of sale figures but on the selections of bookstores, from which they generate edited estimates of rankings.  These published lists don’t indicate how many copies of a book have sold from those bookstores or the relative sales among books on the lists.  Since 2009, BookScan does supply weekly “book charts” to the Wall Street Journal, but they're just best seller lists. 

The bottom line is that only book publishers have comprehensive sales data, and they don’t usually make the information 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 identifies available collections. It is available to Harvard Library patrons in the Loker Reading Room in Widener (call number RR 101.18), at other libraries and via various online bookstores, and the full text of the book can be searched (but not read, for copyright reasons) at HathiTrust.