Alternative Proposals

Option 2: Add an Attribute

The current solution requires users to know there is a different between the first file and remaining files in terms of how sphinx treats it internally.

We could add a frontmatter attribute requirement for the first file to be treated as it is currently by default (as frontmatter)

Current

- file: myintro

- file: firstchapter
- file: secondchapter

Proposed

- file: myintro
  frontmatter: true
- file: firstchapter
- file: secondchapter

Option 3: Domain based Context for toc.yml

The toc is focused on content rather than front page of book which is contained in _config.yml. A useful reference for latex book structure

type: book
front:
  - file: preface.md

main:
- part: My first part
  chapters:
  - file: part1_firstchapter
  - file: part1_secondchapter
- part: My second part
  chapters:
  - file: part2_firstchapter

appendix:
  - file: appendixA.md

back: 
  - file: final_note.md

This would maps directly onto LaTeX structures for the book class.