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You want this:

The Craft of Text Editing

—or—

Emacs for the Modern World

–by–

Craig A. Finseth

And the earlier EMACS: The Extensible, Customizable Display Editor by rms https://www.gnu.org/software/emacs/emacs-paper.html

which is more of a 10,000 foot view.

This is awesome.

I like Finseth's 1980 thesis better than the 1991 book

Theory and practice of text editors, or, A cookbook for an Emacs