I became really interested in PIC late last year. It is near impossible to find information on it online due to the short, generic name. Google certainly has made naming projects for search-ability something to keep in mind.
That last link has esr's rewrite of the pic manual for groff.
So far as I know, that's the best pic manual available (I'm
a heavy user of pic in the past, still use it today, I'm the reason that gnu pic has the ``i''th notation in it so you can write pic docs where you change one variable and it changes the number of items in the picture as well as all the things that point at those items. I can dig up the doc I did that used that if you like).
That last link is W. Richard Steven's website, the Unix networking books guy. I miss that guy, he and I used to trade notes on how to do stuff in groff.
Here are a few other links I collected:
http://www.cs.bell-labs.com/10thEdMan/pic.pdf - The PIC manual. Top result on Google for "bell labs PIC manual" but seems to be down at the moment.
http://figr.bzero.se/ - figr, an online PIC processor
http://www.kohala.com/start/troff/troff.html - some PIC (and troff) resources from the author of "Programming in the UNIX Environment"