One of my all-time favorite books which of course I’ve owned multiple times but never read in its entirety: UNIX Text Processing, which covers the roff family in detail, plus eqn, awk, etc.
https://github.com/larrykollar/Unix-Text-Processing
A re-compilation of the book by various contributors coordinated by Larry Kollar.
The whole thing compiles in a few seconds on an old Thinkpad (I had to fix a font location warning).