Create/insert/duplicate a page of spread

Do you need text as a single thread? 

You can just drag&drop pages between documents - or use move in the Pages Panel. 

There are no automated ways, no. Possibly a script, but unless you need this process fairly often, it would take longer to find one, install it, resolve any problems (because, say, it hasn't been updated in a few versions) and then find out it has some added quirk you don't like, than to...

• Open your Chapter 1 file. Save-As the new book file.
• Go to the end and make sure it begins with a new, empty paragraph; probably set that paragraph style to the leading style in each chapter (Chapter Heading, etc.)
• Open Chapter 2. Select all (Ctrl-A) and copy (Ctrl-C).
• Put cursor on that new paragraph in the new book file, paste (Ctrl-V).
• Repeat last two steps until all content is pasted into the one new file.

Regardless of method, you're likely to have to go through and touch up formatting, especially that concerning smooth page flow. But if all your text is in one flow per chapter, and all your other elements are anchored to the text, and all chapters start in the same way, this five minutes should produce a pretty clean starting point for a one-file book. It's only if you don't have that regular chapter start that there might be flow changes and things like tables or images might need to be fixed up.