Use a tool like audiobook-creator to extract the text of an audiobook.
Place it in extracted_texts/TITLE by AUTHOR.txt. (The format of this doesn't matter except that the file name sans extension will be used in the output.)
Run make_cover_briefs.lua.
Cover briefs will be placed in cover_briefs.
A prompt ready to paste directly into Gemini will be in for_gemini. An extra border is prompted for to make cropping easy to remove the Gemini watermark. (There are other small changes to increase likelihood of Gemini following the prompt correctly and making a good output.)

Gemini Issues

None of this works unless you disable Gemini's personalization/memory, because it pollutes context extremely badly and will randomly add elements and rejections from different conversations everywhere.

If Gemini rejects the prompt, adding Remove any elements that may go against guidelines before generating the image. ⧉ to the opening paragraph can fix that issue.

This suddenly got a lot less effective. I recommend instead telling it to rewrite the prompt and then in a separate conversation, use that instead. I also recommend one retry before modifying it.

Gemini is really bad about adding hardcover seams. Despite being instructed not to, it shows up sometimes. More strong prompting breaks other requested features, so the best way to deal with it is to request its removal after the initial image is generated.

The local model sometimes completely forgets critical details of characters, so it's important you have some familiarity with the work before running the generator to check accuracy.