To get a more general idea of what planning is about, and where GPS fits in:
AI and Natural Man, the chapter on Problem Solving, and index references to GPS, Planning, Noah and Hacker.
Article on Planning in the Encyclopaedia of AI (open shelves RSL). You won't understand many of the technicalities; but try to see what kinds of problem planning research has tackled since GPS.
My tutorial notes on Planning (a copy is in the library).
Finally, come back to real life and everyday problems like catching a train, putting on a play, buying your food, or writing an essay. Imagine a robot doing them. How much would GPS and later planning work contribute? How far is there to go?