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Treatises are the most common secondary sources used by practicing attorneys, but consider also using any of other following types of secondary sources. Regardless of what kind of secondary sources you use, they are great places to start your research because they can
Legal encyclopedias are similar to general encyclopedias like Encylopedia Britannica or Wikipedia, but they are limited to legal topics. They cover a wide range of legal topics and provide very basic and easy-to-read explanations of legal concepts.
The two major legal encyclopedias are American Jurisprudence Second (AmJur) and Corpus Juris Secundum (CJS). The sections in these encyclopedias will include citations to cases and statutes from all 50 states and the federal system. AmJur is available on Westlaw and Lexis. CJS is available on Westlaw.
Many states also have their own state-specific encyclopedias. Illinois has Illinois Law and Practice (Westlaw) and Ilinois Jurisprudence (Lexis).
Law Reviews refer to articles from hundreds of journals published by law schools around the country. Law reviews are well-researched articles that provide in-depth analysis of a narrow issue of law. They contain extensive footnotes that point you to relevant primary authority and other secondary sources.
Lexis and Westlaw both contain law review articles in their secondary source collections. Northwestern also subscribes to Index to Legal Periodicals and Books, a specialty database that organizes legal periodicals by topic. A free alternative for law review articles is Google Scholar, a separate scholarly search service from the Google you're used to using for general research.
American Law Reports (ALR) is a collection of essays referred to as annotations. Each annotation includes an article on a specific topic, similar to a law review article, plus references to cases from many different jurisdictions on that topic. ALR annotations focus on a narrow legal issue, unlike a treatise that will include in-depth analysis but broadly cover one area of law.
An ALR annotation begins with a table of contents outlining the issues covered in the article. The heart of an annotation is an article discussing the issue. After the text of the article, the annotation includes a tool called the Table of Cases, Laws and Rules that lists by jurisdiction the cases discussed in the article.
The Restatements are an attempt to organize or "restate" the common law of the U.S. There are 14 Restatements on a range of common law issues including Contracts, Torts and Property. The Restatements are published by the American Law Institute, a group comprised of leading lawyers, judges and law professors.
Each Restatement is organized into chapters and sections. A section includes a statement of the "black letter law", followed by a Comment explaining the rule, Illustrations describing how the rule applies to a few hypothetical situations and finally a Reporter's Note describing the history of the section with citations to cases.
The Restatements are broadly available on Lexis and Westlaw. You can also find historical material pertaining to the Restatements, their drafting and their prior versions, on HeinOnline as part of their American Law Institute collection.