The Bluebook. Few books cause law students as much dread, pain, anger, and frustration as The Bluebook.
The Bluebook, formally titled The Bluebook: A Uniform System of Citation, is the style manual for citing to legal documents within the United States. It is now in its 20th edition, more than an inch thick, and consists of over 500 pages of guidance on legal citation. Needless to say, it is large, a bit unwieldy, and challenging at first. However, it is an essential book that every lawyer should know how to use.
Most of The Bluebook, however, is irrelevant for many people who create legal documents. You can ignore more than two-thirds of the book if all you need to do is to cite cases and statutes -- which covers most of the first year of law school, and a lot of litigation. In fact, if you're dealing only with cases, litigation documents and laws, you might be able to find the answer to your question using the cheat sheet on the inside back cover.
Links 11/17 (JJM)
Links 6/20 (SLN)
Updated 11/20 (DGD)