For tax memos, your research will typically include a combination of three types of resources:
1. Internal Revenue Codes - These are the tax laws determined by legislative acts of Congress.
2. Tax Regulations - These are the Treasury Department's interpretation of the laws. Some are temporary and others are not. They are all published online in The Federal Registry. But you can more easily search them in CCH.
3. Tax Cases - These are legal proceedings from the judiciary regarding challenges or violations of tax laws.