If a document has been typed in Word 2019, can it be converted by Word 2010? This is a straight book manuscript, no fancy formatting involved. I will, however, need to add formatting (e,g, chapter headings in a different font) once I have the mss. in Word 2010.

Word adopted the DOCX format in Word 2007, so you should not have a problem opening a 2019 file. You might experience some minor issues of the file contains features that are not supported in 2007, but from your description, you should not have any of these.

Make a copy of a file and then open the copy in W2010--let us know what happens.

You can absolutely open and edit a .docx file created in Word 2019/365 in Word 2010. There are some features that will not function in the earlier version, but it does not sound like you are using anything like that.

If you are writing a book in Word, it is very important that you learn to use Styles and that you base your chapters all on the same template using the same styles if they are in different files.
Importance of Styles in Word


I strongly suggest spending a few minutes at least reviewing Basic Concepts of Microsoft Word - from Shauna Kelly. It will save you days or weeks of frustration.