The .indb file is the Book file that links the 20 chapters together. If you open that, you have easy access to all of your chapters, page numbers update across chapters, and all styles and master pages are able to be synced if you wish. When you are finished, you can output a single PDF from the book panel. I cannot think of a good reason to combine these documents. The way it is, you can work with more manageable size files, and if there is an index involved, I think you will find that it works much faster the way it is.
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