I've added a "Design philosophy" section to
https://libprs500.kovidgoyal.net/wiki/Development
and there is a sticky (though in a somewhat obscure forum)
https://www.mobileread.com/forums/showthread.php?t=14083
And I have to say that I've recently been getting a few nice patches. In the next release you'll see support for epub2lrf, epub-meta and the Kindle thanks to patches from third parties.