My wife worked there for many years, as a teacher in the Education Dept. At that time it was a B Cat Training Prison, so had a very wide selction of prisoners. As late as the early 1990s roadblocks at the roads off Dartmoor were common when a prisoner ran off from an outside works party.
I did a small bit of research in 1989, at the time of the 200th anniversary, with a view to helping to put together a model of the prison whilst under construction. This next bit is from memory, so I stand to be corrected, but I think the prison was run by the Transport Office, with prisoners being issued a smock with the letters T.O. on chest and back, presumably when their own clothing wore out. I seem to remember that it was described as being yellow in colour. This might be as means of distinguishing them, or it could be just bleached cloth that took on a yellow tinge.
At it's height as a POW centre it held both continental prisoners and those captured during the War of 1812, mostly sailors from US ships. The two groups apparently didn't get on well. There were clashes, deaths and subsequent executions. There is an area within the walls which holds unmarked graves of prisoners, and some others within the graveyard of the Parish Church, which is outside the walls. There used to be a Stars and Sripes hanging in the church, presented by the Daughters of the Revolution (I think) to commemorate the US prisoners.
The internal organisation seemed to be rather haphazard, with a fair degree of autonomy within the walls. The prisoners appear to have divided themselves into groups (gangs?)with names taken from Roman political structure, like Plebs and Senators, or it could be a hang-over from the earliest prisoners, during the Revolutionary Wars. There was the usual trog group who were rumoured to practice all sorts of dire acts, including cannibalism. As far as I am aware (at least at that time) there hadn't been a detailed history of the prison itself, although it was mentioned in most works on Devon. What we did find out was entirely due to a Prison Officer going back over records held within the place. These records do not appear to be extensive, at least as far as the early years are concerned.
Princetown itself was a "model" new town, named after the Prince of Wales and intended to provide the best that late 18th century town planners could come up with, which doesn't appear to have been a lot, since it was the harshest place the Government could come up with to build a prison. It is the bleakest of places, wet and foggy more often than not, and has a certain unsettling quality about it. On the plus side it has a couple of excellent pubs, one of which brews its own ale, lots of history, from stone circles to abandoned mines and quarries and enough suicidally notioned livestock to make driving a car a real adventure.