How does a retired author report royalty income from past work he is not actively involved with?

From the information you provided, the income from the royalties received on the sales of your books/other media should be reported on Schedule E. This is because you are no longer regularly engaged in operating a business with the primary purpose of profit. ‌

You should choose the first option when entering your 1099-MISC  "Royalty investment income from a natural resource, intellectual property or other investment portfolio." Depending on which option you choose, the income will be reported on a different schedule on your tax return.  The first option, "natural resource or intellectual property" will flow to Schedule E, while the second option, "being self-employed as a writer, inventor, artist" will flow to Schedule C.

Here are the instructions for reporting  How do I enter a 1099-MISC for royalty income?