Word field codes let you insert anywhere in your document several data that can be updated automatically, such as tables of contents.

You can find a list of fields easily by going to Header and Footer Tools > Document Info > Field, or by writing in Command Search the words Add document info, then going to Field.

In the dialogue that opens go to Field names > StyleRef, click on it and in the right pane double click the heading that agrees with your chapters’ titles. If you have titles of multiple levels (Heading 1, 2, etc.) select the heading level(s) of the titles you’d like to appear in your headers.

This inserts in the headers a code representing the current section’s heading, which is your chapter title. Now whenever a new heading of this type (1, 2, or whatever you selected) appears in your document, this particular chapter’s title shows up automatically in the headers of your book and remains until a new chapter begins.

You may add normal text besides field codes, but this text remains the same in all headers of all chapters. Let’s say you have a header in the form: Chapter + FieldCode. The word “Chapter” remains the same throughout.

To have unique words or phrases that won’t be generated by a field code, you need the manual separation (unlinking) of neighboring sections’ headers and footers.