A regulation is issued by the Internal Revenue Service and Treasury Department to provide guidance for new legislation or to address issues that arise with respect to existing Internal Revenue Code sections. Regulations interpret and give directions on complying with the law. Regulations are published in the Federal Register. Generally, regulations are first published in proposed form in a Notice of Proposed Rulemaking (NPRM). After public input is fully considered through written comments and even a public hearing, a final regulation or a temporary regulation is published as a Treasury Decision (TD), again, in the Federal Register. (Understanding IRS Guidance: A Primer)
IRS Written Determinations are documents the IRS is required to make "...open to public inspection..." pursuant to the provisions of I.R.C. Sec. 6110. The documents prepared for release each week are made available to the public every Friday morning. (from IRS Written Determinations)
Decisions of courts, or case law, establish precedent. Case law is one source of legal authority, along with statutes and regulations. Before you rely on a case, be sure to update case law to be sure another case has not overruled it.
To find relevant case law on a topic, start with a secondary source, and mine the footnotes for citations to other sources, including cases, on your topic, Once you have looked at secondary sources, you can also search case law.
If a more recent case questions or even overrules the case you located, citators (Keycite in WestlawNext or Shepard's in Lexis Uni) will let you know. Citators also serve as a research tool - identifying additional cases on your topic.