The Bluebook is the dominant style manual for legal citation in the United States. It is currently in its 19th edition.
Pt 1, the Bluepages, is a basic guide to legal citation designed for practicing legal professionals and 1Ls.
Pt 2 contains more complex rules of citation style required for law journals and other legal scholarship.
Pt 3 consists of tables which show which authorities to cite in various jurisdictions and how to abbreviate them.
A Bluebook app is also available. Download the free Rulebook and the do an in-app purchase for the Bluebook.