The GNU Troff Manual is the on-line documentation by Trent Fisher in Texinfo format included with the groff package. The latest groff can always be found at the Free Software Foundation, http://www.gnu.org.
Troff User's Manual, also knows as cstr54, is the essential and classic documentation for troff, originally by Joseph Ossana and revised by Brian Kernighan in 1992. Available from the Bell Labs archive, http://www.cs.bell-labs.com/cm/cs/cstr.html. Kernighan's 1978 Troff Tutorial can also be obtained from that site.
W. Richard Stevens' web site continues to be maintained and includes valuable inside information on how he used groff for his own books, at http://www.kohala.com/start.