When I press "Regenerate all" on an imported book, where are the new chapters written?

From version 1.9.24, "Regenerate all" on an imported book starts on "In this book", as it does for every other book: the chapters are written again in the same book, and each chapter's current text is kept in its Versions panel, so you can always go back. To keep the book exactly as it is, choose "In a copy of this book" instead: a copy is made, it opens next, and both are in your Library.