The American Law Reports (ALR) is an excellent starting point for U.S. legal research.
It gathers in one place the wide range of legal rules and outcomes from many jurisdictions on a single specific issue. Consult ALR in Westlaw.
American Jurisprudence 2nd Series and Corpus Juris Secundum are LEGAL ENCYCLOPEDIAS. Think of these as an extremely detailed Black's Law Dictionary, with more detailed definitions for terms-of-art and doctrines. Consult these sources in Westlaw.
If you know nothing about a particular subject, and would like a brief and authoritative introduction, then you should use a NUTSHELL.
A great resource to help you prepare for exams is a CONCISE HORNBOOK.
It is not as detailed as a HORNBOOK, and provides a clear explanation of black-letter law and policy.
If you would like a more thorough introduction to a field of law, which includes a historical and theoretical background, then you should use a HORNBOOK. Hornbooks are gold-and-green hardcover books published by West.
If you would like a multi-volume work that comprehensively describes a field of law in exhaustive detail, then you should use a TREATISE.
Treatises may be geared toward either a scholar or an experienced practitioner. West's treatises generally have a more academic bent, while the Matthew-Bender series published by Lexis is geared more towards practice.