15. I have written a book with two other co-authors. The book consists of 12 chapters and each of us is the sole author of 4 chapters. How is the copyright for this book shared between us?

This is a case of distinct authorship. Each author owns the copyright for the chapters which he has written. Each author can exploit his own copyright without having to obtain the consent of the other authors. If the authors wish to have some arrangement about the copyright for the entire book, they have to come to an agreement on those arrangements.