It is possible that for some reason, your reverse gear(s), depending if you have 4WD or not, got grinded away. Check your main gear book on the drive axles. That happened to an old 92 Ford Taurus SHO I had. Good luck!!:)
Haynes is good. Advanced and Auto Zone have them. I think Chiltons is better. Car-Quest has a good selection.
It most likely means they aren't getting any pressure output. I would check the electrical connection to the compresser first and if there is power when the A/C switch is on then it means a problem with the compresser itself. If so, that would mean that the compresser needs to be replaced?
The sending unit is located inside the fuel tank. You need to go to a car parts store, and get a repair manual for your car. They cost about $16.00