I highly recommend AsciiDoc (specifically Asciidoctor)! It has a ton of great features, has standardized syntax (unlike Markdown), can easily include sections of source code from other files (super cool and useful for KDE code documentation), can include other AsciiDoc files to make document management easy, requires no special XML formatting application, and if you follow their recommendations (keeping all sentences on one line each, using ATX sections, etc.) it makes source control via Git a breeze. And you can export to any format, including DocBook.

You can also add keyboard shortcuts, menu navigation, and buttons, like in Phabricator!

I've been using it for about a month now for my day job, and it's been wonderful.

I think it would make the documentation and books projects much more accessible!