Well first of all either it's a PIP or it's a Pegasus but never both. As for the manuals....all of the various Boeing Vol. 2 Operations manuals for the B757/767 have a Chapt. 11 Flight Management-Navigation. Wish I could say something nice about the Boeing material but I'm afraid it's pretty hard to digest IMO. Honeywell does publish a FMS manual for the PIP/Pegasus and as I recall it costs about $75US. Also available are the manuals produced and published by Bill Bulfer (sp?) (CAL pilot) Pretty sure you can get a quick handle on them via Google
Good luck