Page 148 of Physics for Engineers & Scientists has a tug boat pulling a barge with a cable. To quote: "We could take the barge as our body, or the cable, or both jointly. Since the barge and cable accelerate jointly, it will be best to take the barge and cable jointly as our body." However then talks about the pulls ate the beginning and end of the cable as not bring action/reaction forces. Isnt this suddenly taking the barge and cable as separate bodies?
Could well be. They are internal forces.
I don't have a copy of your text but I see your point. When we consider the barge and cable as one body we do this because the two "accelerate jointly". This is convenient because then we can find THE acceleration that all parts of this system must have because they are connected and move together jointly. But the tension in the cable must decrease as we proceed down it's length from the tugboat end to the barge end. The difference in the tensions is exactly what is required to accelerate the cable, which of course is attached to and accelerates with the barge (and tugboat too). But yeah, to talk of the forward end and the rearward end of the cable, and to compute the forces exerted on/by these ends by the tugboat and barge respectively, we pretty much consider the barge, tugboat and cable as separate bodies.
In any event, which one you consider to be your body is totally up to you and doesn't affect any physical outcome.