Donald Knuth has written what are arguably the most beautiful books with the TeX system.
Now I understand that Knuth wrote them in plain TeX, and still uses plain TeX for his current projects. Which, to me at least, raises the question: how is it even possible to write a book of any significant size without the benefit of the LaTeX macros and the CTAN packages, some of which, I believe, will only work with LaTeX?
For instance, how did he do his drawings? How did he define his environments? (Did he?) How did he manage floats?
plain.tex, the former written primarily by Leslie Lamport, the latter by Hans Hagen. And plain gives you in fact more freedom - almost nothing is hardcoded! And there are plain TeX packages out there, too (tikz, TAP for tables, and quite a few others; google
gustlib, for example).