How do I divide my book into parts, such as "Part I: The Ranch"?

From version 1.9.24, open the book's page and hover the chapter where the part should begin. Click the "Add part here" icon on its row (the stacked layers icon). Type the part's title, for example "The Ranch", and press "Add part". The part holds that chapter and every chapter after it, up to the next part. Do the same for each part. BookyAI numbers the parts itself (Part I, Part II), so you only type the title; if the title already says it, such as "Part One", the number is not added twice. Each part shows as a row above its first chapter, with how many chapters it holds.