OK, I found the book. It's PENNSY Q CLASS - Classic Power 5, by E. T. Harley and printed by N. J. International Inc.
On page 37 there's a shot of a Q-2 out of service, and in the background you can see a car that's clearly a flat car with a tender tank mounted on the end (still actually has raised sides, like you'd find beside the coal bunker) with a cabin constructed in the center. (You can only see about half the car.) It's PRR car # 496403 and is labeled on its side FIRE FIGHTING EQUIPMENT. It is not identical to the car in question here, but the method of construction appears so close in concept it's uncanny.
The shot is probably around or just after 1952 - the Q2 is out of service (not only is it obviously disused but the front coupler knuckle is missing) and that corresponds to when Mr. Harley was first assigned there and took the pictures in the book.
In the shot you cannot see the very end of the car either, so I don't know if the small doghouse or cabin is there.. but for whoever might have this book, look at page 37. OR, if there are any well-equipped PRR fans reading this, see if you can track down that car or else provide more information.
-Will Davis