IRS Publication 1281, guide to backup withholding of federal income tax and the IRS "B Notice" requirements (current version of Publication 1281 is the one dated 3-2020). NOTE that Pub. 1281 doesn't show the current Form W-9! Obtain the current Form W-9 (revised October 2018) from the IRS website at https://www.irs.gov/pub/irs-pdf/fw9.pdf.
IRS Publication 1586, guide to the "reasonabe cause" regulations and IRS Notice 972CG for proposed penalties that include filing forms with incorrect or missing Taxpayer Identification Numbers (current version of Publication 1586 is the one dated 08-2018)
IRS AIR System for online filing of Forms 1095-B, 1094-B, 1095-C, 1094-C information returns required under the Affordable Care Act.
IRS Publication 1220 provides instructions and specifications for electronic filing of Forms 1099, 1098, 5498, W-G, 1097-BTC, 3921, 3922
IRS Publication 1187 provides instructions and specifications for electronic filing of Form 1042-S
IRS TIN Matching Program -- free validation service for Name-TIN furnished by a customer, vendor or other payee, available to those who file Forms 1099-B, 1099-DIV, 1099-INT, 1099-K, 1099-MISC, 1099-OID or 1099-PATR. To register for TIN Matching or use the TIN Matching Program, go to https://www.irs.gov/tax-professionals/taxpayer-identification-number-tin-matching The TIN Matching Program is within the general on-line IRS efile system.
IRS efile Services Help Desk: 866-455-7438
Tax treaties PDF copies of income tax treaties between the U.S. and foreign tax jurisdictions, plus Technical Explanation papers that clarify provisions of treaties Tax treaty tables prepared by IRS Look-up tables that were in IRS Publication 515 in past years are now available only on the IRS website, plus the IRS now displays an LOB Table covering Limitation on Benefits provisions found in tax treaties
EFTPS Customer Service for federal tax deposits: 800-945-8000