"Customer discovery is the initial and iterative process of understanding customers’ situations, needs, and pain points. Customer discovery involves defining and prioritizing personas and is applicable to both early-stage companies and big companies when developing new products, seeking to target new personas, or entering new markets" (source).
Below are sources of secondary information that can help you discover and/or refine who your customer is, their opinions and attitudes, where they are, and more. Information about consumers is often grouped by demographic or psychographic information
Most of these resources assume your business serves consumers as opposed to other businesses. If your business serves other businesses (B2B), you'll want to utilize information found under the Competitive Intelligence tab for your customer discovery.
American Customer Satisfaction Index
The Index measures the satisfaction of U.S. household consumers with the quality of products and services offered by both foreign and domestic firms with significant share in U.S. markets.
US Census Bureau
United States population demographic information from the U.S. Census Bureau.
Census Business Builder
The Census Business Builder is a suite of services that provides selected data at the county and city level tailored to people looking to start a small business; includes socioeconomic and demographic data, key industry statistics, and even consumer spending data!
Consumer Expenditure Survey
Bureau of Labor Statistics Consumer Expenditure Survey includes "information on buying habits of America's consumers."
Pew Research Center
Contains survey reports, demographic studies, and data-driven analysis.
United Nations Demographic Yearbook
Data collected from U.N. national statistical authorities.