Under “Document” → “Indices and Directories” → “Associated Documents”, you can assign co-documents to your main text, which will then also be opened when you open the main text.

Associate documents with your main document which will then be opened anytime you open the main document

These “Associated Documents” are also always available as a “favorite” under the “Document” menu.

If you have not selected the checkbox next to a document, it will not open automatically every time you open the main document, but you will still be able to access it quickly under “Favorites” in the “File” menu.

Projects are a good foundation for a book, as they automatically generate useful associated documents

Associating documents with your main text allows you to quickly and easily organize texts together so you can find anything related to your main text, be it pictures, videos, songs, etc.