Research Tip: Use an annotated version of the Code (USCA or USCS) for references to relevant case law and helpful secondary sources.
The Code is reprinted in other sources as well, see esp.:
Mertens Law of Federal Income Taxation (available only on Westlaw). Contains the entirety of the IRC, in additon to a wealth of secondary materials.
IRC on WestlawNext – this links only to Title 26 of the US Code and applicable tax-related court rules, which makes it a very good place to go for a more targeted search of tax-related materials.
IRC on GPO Access (free!). PDF and HTML versions of the current and historical versions of the Code.
Research tip: CFR section numbering for tax regs incorporates IRC section numbers, so regulations relating to, for example, IRC § 501(c)(3) are found at 26 CFR § 1.501(c)(3).
In addition to the CFR in print, Treasury regs published in the CFR may be found in many other sources, including the following:
CFR on WestlawNext – this database contains only Title 26 of the CFR, which makes it a very good place to go for a more targeted search of tax-related regulations.
CFR on GPO Access (free!).
Other Official Tax Guidance: Internal Revenue Bulletin / Cumulative Bulletin
The Internal Revenue Bulletin (IRB) is the official source for publicly-released, tax-related documents, including: treasury decisions, revenue rulings, legislation and treaties. IRB is published weekly and compiled twice per year in the Cumulative Bulletin.
For brief definitions of the types of tax documents published in the IRB, see the Hierarchy of Tax Authories tab in this guide.
Research Tip: These IRS documents are also available in individual databases within Lexis and Westlaw. So, for example, if you are only looking for a General Counsel Memorandum, you should search within the smaller database (examples available in the tab entitled "Heirarchy of Tax Authories") for more precise results.
IRB on the IRS website.
Westlaw (FTX-RELS) – includes IRB, Cumulative Bulletin, Private Letter Rulings/Technical Advice Memoranda, General Counsel Memoranda, and Technical Memoranda, though dates of coverage may vary.