The practice centers listed below are a great place to start your research. Westlaw, LexisNexis, and Bloomberg Law each have comprehensive resources that you can use to research your client's issues.
Westlaw offers a great tax practice center with access all the case law, annotated statutes, regulations, and secondary sources you could hope for in the area of tax practice.
Notable Secondary Source: Leslie Book & Michael Saltzman, IRS Practice and Procedure, (rev. ed.) Weslaw.
For tax procedure, the Saltzman & Book: IRS Practice & Procedure is a great treatise to use in beginning your research. . Compared to other resources, this treatise offers a relatively concise table of contents, organized around procedural functions and key issues. The Saltzman & Book treatise provides a great overview of the law and includes relevant statutes, regulations, major cases, and Revenue Rulings to each topic.
For researching more substantive tax issues, Westlaw offers a bevy of secondary sources, including:
Lexis also has a tax practice center page which will direct you to helpful secondary sources, and search for tax related statutes, regulations, administrative materials, forms, and news. They also offer the "Lexis' Tax" page which allows you to filter through material relevant to federal, state or international tax law.
Notable Secondary Source: Mark Johnson & Jacob Rabkin, Federal Income, Gift and Estate Taxation, (rev. ed.) Lexis.
Both substantive and procedural tax law can be found within Lexis's Federal Income, Gift and Estate Taxation treatise. Using the link, you should see a huge Table of Contents. Chapter G provides you a guide to using the resource, which also provides a helpful section on the various weights of authority to the sources which are cited to. Additionally, this treatise includes checklists and will either provide examples or point to the relevant tax forms you may need.
Tax procedure starts around chapter 74 of the treatise and goes until the end at chapter 88. You can also find a checklist in chapter 77 which gives a quick guide to weight of authority given to regulations and rulings.
Bloomberg Law's tax center is its own site, which provides you access to its incredible BNA Portfolios, primary resources, guides, checklists and forms. Bloomberg's search function is also well optimized. You can easily find relevant portfolios to your taxpayer's issues by using a natural keyword search through the main page for a greater range of coverage or, if you want to be more specific, within the portfolio groupings themselves.
Notable Secondary Sources: U.S. Income Portfolios: Procedure and Administration, Portfolio 619 - 648, Bloomberg Law.
If you are unfamiliar with federal tax procedure, you will want to start with their U.S. Income Portfolios. Their tax portfolios on "Procedure and Administration" will provide you with a very detailed "how-to-research" guide, how to interpret various authorities, and what weight to give them. Topical coverage inside the portfolios is well organized and very thorough.
Navigation
Westlaw Edge's tax practice center can be found by navigating to the "Practice Areas" tab on their homepage and clicking on the "Tax" hyperlink. If you want to start with a good secondary source, you will want to click on the "Warren Gorham & Lamont (WG&L)" link underneath "Tools" on the right-hand side of the webpage. From here, you will want to click on the "Tax Practice & Procedure" hyperlink. Here you will find a list of four treatises. The one you will want to start with is "Saltzman & Book: IRS Practice & Procedure."
Tools and Other Functions
You can do a lot from the Tax practice center home page. Under the tax resources section, you can select content that you want to limit your searches to, including tax forms. Under the "Tax Analysis" section, you can find selected tax news and journal articles from Westlaw Today. You can also find news on federal tax issues from Checkpoint, but these require a Checkpoint subscription.
Under the "Tools" section to the right of the page, you can open links to search for tax documents, tax news, and find additional secondary sources through the "Warren Gorham & Lamont (WG&L)" link.
Navigation
Lexis+'s tax practice center can be found on the homepage by clicking "Tax Law" underneath the "Practice Areas" tab. From here, you can scroll down the page the "Secondary Sources" and click on the "Federal Income, Estate and Gift Taxation" link, which is currently listed first underneath the "Top Secondary Sources" drop-down menu. This will take you to a treatise which covers both substantive and procedural tax issues. Tax procedure is covered near the end of the treatise, from chapters 74 through 88 in the Table of Contents.
Lexis also offers a tax specific page which you can navigate to by clicking on the top-left 9-dotted box and clicking on "Lexis' Tax." This will bring you to a page which organizes all the primary and secondary sources relevant to federal tax law. You can also click on the Lexis Tax Advisor - Federal Topical (TAFT) to find another great secondary resource on federal tax procedure. Part 5 of the TAFT deals specifically with tax practice, procedure and controversies, but if you are just getting started, use the Federal Income, Gift and Estate Taxation treatise for more thorough coverage of the topics.
Tools and Other Functions
Both of these pages offer similar tools, such as links to primary and secondary resources, tax form finders, and access to current news. Lexis' Tax page will actually let you drag and drop the sections around the page so you can organize them to you preferences and add sources to your own "favorites" group.
Navigation
The Bloomberg Tax page is found by navigating to the "Practice Centers" drop-down menu on their homepage. Clicking on the "Tax" link will take you to their tax specific website. From here, you will want to navigate to the "U.S. Income" portfolio, found under the "Expert Analysis" box. Once in the U.S. Income portfolio, you will want to expand the "U.S. Income Portfolios: Procedure and Administration" drop-down menu in the Table of Contents.
Tools and Other Functions:
The Bloomberg tax page offers links to primary sources near the bottom of the page, along with their portfolios under the "Expert Analysis" section. You can also navigate to the "Practice Tools" section to find tax forms, checklists and other guides. If you scroll down the page, you can find links to their Daily Tax Reporter and Federal Tax Development's tracker to find the latest articles and news on tax law.