There are two types of legal research sources that are particularly good for starting tax research--treatises and looseleaf services. Treatises offer expert analysis of tax issues and can help with understanding the major points of law on a topic. Looseleaf services are a hybrid of primary and secondary sources, compiling statutes, regulations, excerpts from IRS documents and tax court cases with analysis. They are typically organized by tax code section, and provide an easy way to find most or all of the available primary source material related to that section. The Legal Research Center primarily provides online access to these services, rather than the print binders from which the name comes.
Tax law information comes from a number of different and often unique documents that have a fairly strict hierarchy of authority. When doing tax research, it is important to understand where the information that you've found fits in the hierarchy, along with the other usual factors (such as whether the authority is primary or secondary, or what jurisdiction the authority comes from).