Can I decide what each story in my collection is about?

Yes, from version 1.9.20. When you create a collection, the Structure step of the New Book form shows "Your stories": one row per story, with a title and a line for what it is about. Both are optional. Fill in every row and the book follows your list exactly, story by story, in that order. Fill in only a few and BookyAI invents the empty ones in the same spirit, each one different from yours and from each other. A row with only a title gets a story that fits the title; a row with only the "what it is about" line gets a title that fits the story. After the outline, what you wrote for a story becomes that chapter's Instructions, which you can read and change on the book's page before or after it is written.