Adding Files to a Book
If you use InDesign’s Book feature to work with long documents, you probably know that you can add individual InDesign files to a book by choosing Add Document from the Book panel menu or clicking the plus sign at the bottom of the Book panel.
But you might not know that you can also add files to a book by drag and dropping them into the Book panel from the Finder on the Mac, Windows Explorer, or Adobe Bridge.
You can drag and drop multiple files at once, or even a folder.
InDesign adds all the files (including any in nested subfolders) to the book.