Pandoc seems to handle putting multiple Markdown files into an ePub file as if they were all of one document, as the table of contents of an ePub that I generated using Pandoc listed all instances of H1 headings as top level items, rather than making them sub items of a top level item that accorded to each individual file inputted. One file included multiple H1 headings, and the headings were all represented in the TOC.

I used the guide here that shows how to generate an ePub file from multiple files, and I learned that I can place the input files' names all on one line — I don't need to break the file names onto multiple lines, with a backslash to indicate that the same command is still proceeding on (or at least I think that's what the backslashes are for).

This is all in an effort to produce ePubs prior to pushing any changes of one of my books to Leanpub — letting me stay on their Free tier.