With the newpagetemplate plugin, it looks like what is intended to happen is that I use that plugin to create a separate page (one page for each instance of a transclusion [so one page for each book to continue the earlier example]), which then needs to be included into the main page. Is that correct? If that's the case, it's not really ideal either.
If I understand you, yes. But so, too, does the templater plugin create separate pages for each instance of inclusion. Its advantage. obviously, is the greater immediacy, that is, the included data is immediately inserted into the current page. But there are advantages to newpagevars if you use the openas plugin as a front end to creating of your inclusions.
1. The same form created by openas can be used for more than one template, because you can attach more than one SAVEAS_PAGE button to the form, and each button would access a different template. So if you give the form variables names like a1, a2, etc, you can include the same macros in more than one template: @a1@, @a2@, etc. This means, for instance, that you could use the same form for a two column table and a three column table.
2. The form sends its output to the newpagevars plugin, which creates a page with the macros filled in, and since the newpagevars plugin ignores macros which have no data, you can create a form and templates with the maximum number of fields you might require. The input fields which have not been filled in will have no effect on the output.
3. For me, it's the convenience that I like. It's not necessary to enter either a url (newpagevars) or markup (templater) with a string of names and values and their separators. Particularly convenient when you have a significant number of macros.
After the page has been created you can use the include plugin to insert it into the master page.