I would use AsciiDoc for writing the content as it supports the Simple DocBook functionality. AsciiDoc converts natively to HTML, and converts to epub and pdf through its a2x helper.
For PDF output there are two options, either use DocBook+FOP, or asciidoc-odf to produce PDF output styled through LibreOffice.
Since AsciiDoc is used nowadays for various O'Reilly books it is up to par with the requirements for printed material and digital formats.