Whether someone is classified as an employee or an independent contractor has important legal ramifications for both labor law and tax law. As such, one must be careful to evaluate which context a secondary source is coming from.
Bloomberg Law has a number of employment law treatises, all available from the Labor & Employment Practice Center. Particularly useful ones in this area are Wage and Hour Laws: A State-by-State Survey, and the Wages and Hours Manual.
For a tax perspectice, the best place to start is in the BNA Tax Portfolios, specifically Portfolio 391-4th: Employment Status — Employee v. Independent Contractor and Portfolio 399-2nd: Employee Benefits for the Contingent Workforce.
WestlawNext & Lexis Advance
Both WestlawNext and Lexis Advance have a number of treatises on both tax and employment law. As this issue can arise in so many contexted, searching broadly is a good way to begin. Do not forget that these services may also have state-specific secondary sources on this issue as well.
WestlawNext has a Wage & Hour Law: Compliance and Practice treatise with a chapter dedicated to the issue.
The article Establishing Employee or Independent Contractor Status in American Jurisprudence: Proof of Facts has many citations to cases where this issue has been litigated.
Searching within the American Law Reports series could turn up summaries of case law on narrow independent contractor / employee issues.
Internal Revenue Service
The IRS provides a short reference on Independent Contractor or Employee. This is not reliable legal advice. However, it does provide a framework and good terms of art to form the basis of a search query on the topic.