I corrected a misspelled word, but it came back when I regenerated the chapters. Why, and how do I fix it for good?

The AI writes each chapter from more than the chapter itself: the outline, the Codex (characters, notes and the stories you told in an interview) and short summaries of the chapters before it. If the misspelling is in any of those, a regenerated chapter brings it back. From version 1.9.23, "Replace" on the book's page (next to Codex) changes the word in all of them at once, not only in the chapters: the outline, the Codex, your notes and interview stories, the book plan, the chapter titles and instructions, and the chapter summaries. After that, regenerating no longer brings the old word back.