Groff: The GNU Implementation of troff, by Trent A. Fisher and
Werner Lemberg, is the primary groff manual. You can browse it
interactively with 'info groff'.
'Troff User's Manual' by Joseph F. Ossanna, 1976 (revised by
Brian W. Kernighan, 1992), AT&T Bell Laboratories Computing
Science Techical Report No. 54, widely called simply 'CSTR #54',
documents the language, device and font description file formats,
and device-independent output format referred to collectively in
groff documentation as 'AT&T troff'.
'A Typesetter-independent TROFF' by Brian W. Kernighan, 1982,
AT&T Bell Laboratories Computing Science Techical Report No. 97,
provides additional insights into the device and font description
file formats and device-independent output format.
groff(1)
documents the -Z option and contains pointers to further
groff documentation.
groff(7)
describes the groff language, including its escape
sequences and system of units.
groff_font(5)
details the device scaling parameters of device DESC
files.
troff(1)
generates the device-independent intermediate output
documented here.
roff(7)
presents historical aspects and the general structure of
roff systems.
groff_diff(7)
enumerates differences between the intermediate output
produced by AT&T troff and that of groff.
gxditview(1)
is a viewer for intermediate output.
grodvi(1), grohtml(1), grolbp(1), grolj4(1), gropdf(1), grops(1),
and grotty(1) are groff postprocessors.
