How do I give my book standing instructions that every chapter follows, now and later?

From version 1.9.23, open the book's page, then Book details, and write them in "Instructions for every chapter". Every chapter written or written again from then on follows them: a regeneration, "Regenerate all", "Extend with AI" and new chapters. You can also set them from the "Regenerate all" window by ticking "Keep these for this book". To stop using them, clear the field in Book details. They work like the instructions you can give a single chapter, but for the whole book.