Can I write my own prompt for the cover instead of letting BookyAI decide?

Yes, from version 1.9.7. In the book's workspace, the cover card has a "Cover art direction" choice: "BookyAI decides" (the genre-based art direction, as before) or "My prompt". With "My prompt", describe the cover you want in the text box: the scene, the style (painted, photographic, flat illustration), colours, mood, what must and must not appear. Your description replaces BookyAI's art direction; tick "Also apply BookyAI's genre art direction" if you want it kept as secondary guidance. Click Save brief, then Generate or Redo. The title and author name are typeset for you and the technical rules for a flat 2:3 cover with no extra text are added automatically. The same choice exists in the Cover Studio.