There are no 'fundamental theories' to pic programming really... Just practical examples, of which there can be found an absolutely huge amount of if you simply Google for PIC programming examples.
If you want basic understanding of a PIC from the ground up download the FULL PDF for the chip you're using also download the PDF for the PIC ASM basics, and read the other vastly useful instructional PDF's that can be found on Microchips website and on the net at large as well.