The Chassis Service Manual has all the information regarding powertrain, electrical and suspension systems, as well as brakes, fuel,.exhaust etc.
The Fisher Body Manual contains information for doors, windows, trim (exterior and interior ), seats and body electrical.
Those two books will cover any service or repair procedures you are ever likely to need.
Don't waste your money on Haynes or Motor repair manuals; they are too generic.
The GTO Restoration Guide by Zazarine and Roberts is also very useful.
1968 Pontiac GTO
1983 Pontiac Bonneville (G) wagon
2008 Pontiac G8 base