How do I change or remove my book's subtitle?

From version 1.9.17, open the book and move the mouse over the subtitle under the title: a pencil appears. Click it, type the new subtitle and press Enter. To remove the subtitle, empty the field and press Enter. If the book has no subtitle, move the mouse under the title and click "Add a subtitle". Also fixed in this version: a file name, for example of a recording used with BookyAI Listen, no longer ends up as the book's subtitle.