I would like to clarify that the IRS’ rule for royalties not being considered self-employment earnings is only in the case of an author who wrote only one book and never revises it. Otherwise, royalties for books created while the taxpayer was engaged in the business are still considered self-employment earnings reported in sch C, even if the author already retired and no longer writes books.
Ref: Tax Issues for Individuals Who Create Intellectual Property (thetaxadviser.com)
https://weblibrary.thetaxbook.net/thetaxbook-federal-2023/page-5-19)
