Itasca will pay you for sales of your book. Itasca must report those payments to you as income on a 1099 form at the end of the year for you to use in filing your taxes, and at the same time report the income to the IRS.
We need the W9 filled out in order to track payments to you and report them on the 1099. We must have the W9 from you before we make any payments. (The W8 is for publishers outside the U.S. using Itasca.)
The W9, W8, and 1099 forms have nothing to do with sales tax.