troff got me my first job in the industry, working at SCO in the '80s
as a typesetter. I swore by my copy of Word Processing on the UNIX
System by Morris Krieger, and I'm shocked to not see it listed at
http://www.troff.org/books.html but a Google search shows that used
copies are still available, and at reasonable prices.
If you want to go the plain TeX route, I _strongly_ recommend Arvin
Borde's books, TeX By Example and Mathematical TeX By Example. The
books' conceit is to show typeset text on one page, and the source code
used to generate that text on the facing page--such an obvious idea.
Those, plus The TeXbook, are sure to help someone become a solid wizard.
