www.irs.gov includes various publications that provide useful information. For specific information, please refer to the following publications:
IRS Publication 17 - Your Federal Income Tax
IRS Publication 213 - Check Your Withholding
IRS Publication 505 - Tax Withholding and Estimated Tax
IRS Publication 515 - Withholding of Tax on Non Resident Aliens and Foreign Entities
IRS Publication 519 - U.S. Tax Guide for Aliens
IRS Publication 525 - Taxable and Non-taxable Income
IRS Publication 919 - Is My Withholding Correct
www.uscis.gov contains latest I-9 form with complete instructions on completion of the I-9 form.
www.revenue.state.il.us/Publications/otherPub.htm includes publications that provide useful information.
www.revenue.state.il.us/Individuals/new.htm contains new information regarding individual state income tax withholding for individuals.