For anyone missing the lede here: the book is typeset in troff. This is astonishing because troff is 40+ years old and AFAIK hasn't gotten significant development in ~10 years. If it ain't broke don't fix it. http://www.troff.org/history.html
He answered that he still preferred to use groff, having tried alternatives. Since then I have used groff exclusively for writing documentation and papers. It never runs out of gas, and always produces attractive output. It offers a complete suite -- math, tables, pictures, graphs -- all in one package. It's fast, adaptable, and easy to learn. It has the highest ratio of text to markup of any typesetting system, including TeX and Markdown. It is the single most overlooked and under appreciated project on the planet.
Whether it's had "significant" development in the past 10 years depends on what that means. Current groff produces PDF format directly, without a Postscript intermediary. The PDF macros support links and TOC generation. I think that all took place in the last few years.
I have been looking for substitutes to Latex ever since writing my dissertation in Latex and having to debug/tweak an already horrendous Latex .sty file. I basically decided that I would either write technical documents in Tex or not at all after that. For me, Latex is great, until it isn't, and then it's Jinga-on-a-unicycle frustration.
You can always go back to the basics and use simple TeX for your documents if you think groff/troff is simpler.
However, I'd encourage you to give ConteXt http://wiki.contextgarden.net/ a try because it is the spiritual successor to TeX (one tool for all the typesetting) instead of having to depend on "third party" stuff like LaTeX, most of which very few of us (outside the original sty author) understand.