How do I edit the preface, introduction or another appendix of my book?

In the book's workspace, find the section in the "Include in book" list and click Edit next to it. The section opens in the same editor as a chapter: you can rewrite a selection with AI (improve, expand, shorten, simplify or a custom instruction), insert images, restore an earlier version from Versions, and click Regenerate section to have BookyAI write just that section again (your current text is saved to Versions first). The section's heading is part of the text itself, so you edit it directly in the editor.