All book sales. Not just bestsellers, but any book. If possible over time as well to watch as things become popular. A window into what people are reading would be fascinating to me.
Thanks for the tip about Bookscan! According to Wikipedia BookScan's US Consumer Market Panel currently covers approximately 75% of retail sales. That's pretty wide coverage, which is just what I had in mind. Amazon had occurred to me, but I didn't think they would have a large enough percent of total sales.
