Before you update a structured book the first time, each file in the book is represented by an element called BOOK-COMPONENT. When you update, the element for each structured file changes to the tag of the highest-level element in that file—typically a tag such as Chapter that describes the file as a whole. (If a file has more than one flow, the tag comes from the highest-level element in flow A.)
The elements for any unstructured files in the book are still called BOOK-COMPONENT. To change BOOK-COMPONENT to an element tag, add structure to the files and then generate and update again.