ATSG ( Automatic Transmission Service Group ) manuals are used by Trans repair shops, they make good manuals by bringing factory information and some real world fixes together.

However, it isn't a how to manual for a person that hasn't built a auto trans before, there is a certain amount of assumed knowledge. This trans probably isn't one you want to learn on, a 3 speed Chrysler Torque flight , or Ford C4 / C6 would be a better choice. If you do take it apart and bail out, don't even think about taking the pile of parts to a shop and expect them to fix it. Sifting through a jumbled pile of parts takes a lot of time.

Look in the manual for a apply chart, see if any clutch / band elements are used in other gears, if the other gears work, you likely have a valve body problem.

Is this a electronic or hydraulic shift trans?

Did reverse suddenly stop working or was it a slow decline?