At present it uses asciidoctor[1] to go from asciidoc to docbook, then to .docx via pandoc[2] and .pdf via a Manning-provided tool (for styling reasons).

pandoc is pretty incredible. As you can see from its page, it can produce epubs and many other things - it's likely that with pandoc and a docbook file you can build a pipeline to take you wherever you need. For your purposes, you'd probably go from EPUB to kindle with KindleGen.

[1] http://asciidoctor.org/ [2] http://johnmacfarlane.net/pandoc/