The regular FSM has really good info about voltages and whatnot. However the body FSM has the best visual wiring info, with pictures and 3D breakouts and lots of other info. When I installed the automatic day/night mirror I used the body FSM for most of the info.
Of course the FSM is the best. You can get these just as cheap on E-Bay as previously said.
I suppose if you where just planning on fixing small problems and keeping up the maintaince a Haynes or Chilton would do just fine. Personally flipping through them the Haynes manual has a little better layout in my opinion. Chilton has various information spread throughout in a hard to find again type format. At all the same prices I would go with the FSM if you can find one on the net used.