There are several databases you can use to search for secondary sources. In Westlaw, there is the main Secondary Sources page. There is also a page for Law Reviews & Journals, Texts & Treatises, and Jurisprudence & Encyclopedias. Westlaw also has Smolla & Nimmer on Freedom of Speech. In Lexis, there is the main Secondary Sources page. There is also a page for Treatises, Guides & Jurisprudence, and Law Reviews & Journals. See the Cases and Statutes page for tips on searching.
Another helpful resource is The Constitution of the United States: Analysis and Interpretation. Its analysis includes the 1st Amendment.