Good article from "The Tax Advisor" in part " Even though an individual is retired and not currently involved in his or her creative pursuit of income, any royalties received are business income if the individual was engaged in the business at the time the material generating the royalties was produced. 7 In summary, royalty income should be classified as business income for individuals who were in the business at the time the intellectual property was created". Here is the link:

https://www.thetaxadviser.com/issues/2013/dec/kelley-dec2013.html