Blacks Law Dictionary (Via Westlaw, US Tab)
American Jurisprudence (Via Westlaw, US Tab)
Corpus Juris Secundum (Via Westlaw, US Tab) - Both works provide a topical overview of US law with references to support. CJS is fundamentally an encyclopaedic restatement of American case law, citing Federal and State cases with some reference to Federal and state statutory law. Am Jur 2d lists only selected case references with emphasis on Federal statutory laws. Both encyclopaedias contain over 400 titles on a broad range of legal topics.
The Oxford Companion to American Law
The Oxford Companion to the Supreme Court of the United States
The Oxford Guide to United States Supreme Court Decisions
Restatements (Via Westlaw, US Tab) - 'Restate' the common law rules on a subject and also provide commentary on the proper interpretations of the rules, demonstrating how the rules should apply in certain situations, and summaries of cases applying and interpreting the Restatement.