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Assuming, as theducks said, that each chapter is its own file, and assuming that you have the books in epub or azw3, what I would do is:
1. Take the 15-chapter sub-book, open in editor, and highlight the 15 chapter files, then export them (right-click the selection, then export).
2. Open the main big book, and Menu-->File-->Import files into book. Point to your 15 exported files.
3. Drag the files from the bottom of the list, where they will appear, to where you want them among the original chapters.
4. Check that they look OK in the preview. If they don't, the books probably have different styling, css, etc. You can fix that, maybe import the smaller book's css file. This could be no problem or a big deal, depending on the books.
5. Use the ToC editor to do whatever you want with the new files in the ToC. Add them to the ToC and call them whatever you like.
(If the chapters are not in single files, your best bet is to split them up to make it so before you start. Or just deal with chapters as headings in huge files--ugh.)
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