So where to start then?
At the top...

The PBP manual has a section in the back about interrupts in ASM, Read it along with the code.
Basically, when an interrupt is called things have to be saved so the program can pickup where it left off while dealing with the interruption. Have you ever received a phone call and when the call was over forgot what you were doing when the call came in? We do not want the PIC to have that problem.