Can the AI build the character list for me from a book I already wrote?

Yes. Open the book, click "Codex" above the chapter list, and in the Characters tab press "Extract from manuscript". The AI reads your finished chapters and drafts a character card for everyone who acts or speaks in the story — name, age if stated, personality, fears and more. It works on imported manuscripts too. Anything you already filled in by hand is never overwritten: extraction only adds new characters and fills fields you left empty.