Do you need to know statistics or figures about the wages/salaries of workers in the books store retail industry? The first place I’d check is the Bureau of Labor Statistics. They have data on a variety of retail sectors, as well as specific statistics about bookstore workers. The tables for each industry contain occupational employment and wage estimates that are calculated with data collected from employers of all sizes.
The Occupational Outlook Handbook profile for retail sales workers may be useful to those researching this topic.