16. I have written a book with two other co-authors. None of us is the sole author of any part, as we all contributed to the writing and revising of each chapter. How is the copyright for the book shared between us?

This is a case of joint authorship. The copyright for the book is owned collectively by all the authors in equal shares. Any exercise of the copyright must be by consent of all the authors, in particular, any licensing of the book requires the licence of all of them.