Azerbaijan is part of the Comm. Independent States = 0% tax on copyrights if you submit W-8 form with your Tax ID
Those countries to which the U.S. U.S.S.R. income tax treaty still applies: Armenia, Azerbaijan, Belarus, Georgia, Kyrgyzstan, Moldova, Tajikistan, Turkmenistan, and Uzbekistan
source: https://www.irs.gov/PUP/individuals/international/Tax_Treaty_Table_1.pdf