Is there a guided tour for Author voice, Interview Mode and "Plan my book with me"?

Yes, from version 1.9.18. Click "Guide" at the bottom left. Under "Make it yours · optional" there are three entries, each with "Show me how": "Teach BookyAI your voice", "Tell a story of your own" and "Agree on the plan of a book". They tick by themselves once you have an author voice, a story told in an interview, or a book that follows a blueprint. In the list of tours, "Interview me, step by step" and "Your stories and your plan, step by step" open the interview and the planning windows on an example (the book of a baker) and explain every part. In these tours nothing is sent to your AI engine and nothing in your book or your form changes. The Authors tour explains the Author voice card step by step.