The cool kids are using groff -mom of course, but if anyone other than me still uses groff -mm then read on.
man(1) pages are well and good but they're organised alphabetically so it can be hard to get at what you need. Then there are books at http://troff.org, but sometimes you just need a quick reference. These are organised by concept - page formating, list formating etc for me, mm macros and groff.
Source code: https://sourceforge.net/projects/groff-quick-ref/