Historical dictionaries, encyclopedias and biographies are essential reference sources. Consult them to confirm "common knowledge", discover affiliations and related topics, and utilize their bilbiographies to find additional sources, both primary and secondary.
Noticing the subject headings assigned to books related to your topic will help you brainstorm keywords for furhter searching. Think of every synonym you can for any term related to your topic.
business, enterprise, business enterprise, employer, employee, employment, work, worker, job, career (NB an * is a truncation you can use for keyword searching. e.g. employ* tells the catalog to search for the terms employER, employEE, employMENT etc.)
african american, negro, black, minority
specific occupations: barber, tailor, banking, insurance, record label,