Datasheets & Application Notes and Bootloader
These things are essential; The datasheet explains how all the peripherals work, which pin does what, and describes the instruction set of the processor. The errata sheet explains what features described in the datasheet don’t work – nothing is perfect, and bugs do exist, even in hardware. Software engineers are not the only people who make mistakes!
Application notes show examples of how certain aspects of the processor are used; sometimes, they provide full source code, which you can use yourself. The bootloader is an excellent example.
Here is a list of the absolute minimum references that you should have on hand.
LPLC specific resources
LPLC-TOO specific resources
There are hundreds of application notes available for the PIC18F processor family, notes that are ‘generic’ and not specific to any processor type. You can see these here:
On the above page, start by clicking on “PIC18” in the left hand column, then “search”. You can refine the search further by clicking on entries in the other columns.