Circular E, Employer's Tax Guide
http://www.irs.gov/pub/irs-pdf/p15.pdf
W-4 Employee Withholding Allowance Certificate
http://www.irs.gov/pub/irs-pdf/fw4.pdf
W-9 Request for EIN Number and Certification
http://www.irs.gov/pub/irs-pdf/fw9.pdf