Maybe also of interest:
Document Formatting and Typesetting on the UNIX System (ISBN: 9780961533625)
Document Formatting and Typesetting on the UNIX System: GRAP, MV, MS and TROFF (ISBN: 9780961533632)
Some parts are also available at google books.
there’s some more titles on http://www.troff.org/books.html
A free ebook for writing manpages, Practical Unix Manuals, by Kristaps Dzonsons (the author of mandoc): https://manpages.bsd.lv/
It also contains “The History of UNIX Manpages”, another nice read.
This is a scan of the original book. The Groff community transcribed the scans into troff source and generated a new PDF from that, which is much nicer to read: http://home.windstream.net/kollar/utp/
yes, see the last link on the O'Reilly site
I saw, but it was worth pointing out because people are more likely to click “A single PDF file via HTTP” than “groff and PostScript files‐‐Beta”…