Federal Income Taxation: Westlaw

Mertens Law of Federal Income Taxation is a widely-cited tax treatise covering federal income tax law.

Federal Tax Coordinator 2d is a treatise that combines detailed analyses of specific tax issues including income, estate, gift, excise, FICA and FUTA.
The RIA-FTC database contains the Research Institute of America's Federal Tax Coordinator (FTC). The database includes full text coverage of the FTC's topical analyses of federal tax issues. A document is a FTC numbered paragraph.

RIA United States Tax Reporter is a comprehensive reporting service that provides statutes, regulations, cases and rulings in brief, with explanations and commentary. Organized by IRC section number.

Federal Taxation of Income, Estates, and Gifts, by Boris I Bittker and Lawrence Lokken, is a treatise on Federal law governing taxation of individuals and entities.