For Industry & Market Research:

• IBISWorld: Market research reports on the book publishing industry (for US, UK, China and Australia).
• Statista: Dossiers on book publishing
• eMarketer: Reports on accessing media (reading books and published content by type of device, etc).
• Technavio: Reports on a variety of global industries including publishing. Examples of reports: Global Consumer Book Publishing, E-books Market, Global Content Publishing Market, Interactive Children's Book Market.

Note: Baker Library does not have access to sales figures for specific book titles. 
Nielsen BookScan reports point-of-sale data, which represents approximately 75% of all retail book sales.  BookScan is a very expensive subscription service, used primarily in the industry, and Harvard does not have a subscription.

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 make it public.

If you are looking for statistics for books published prior to 1996, publishers' archives are the only reliable source.  A Guide to book publishers' archives, identifies archival collections.