The following documents will provide help with the Microchip C18 Compiler:
c:\mcc18\doc\hlpC18ug.chm
Describes the technical details of the C18 compiler, such as data types etc.
c:\mcc18\doc\hlpC18lib.chm
Gives an overview of the MPLAB C18 library files and precompiled objects files that can be included in your projects
c:\mcc18\doc\MPLAB-C18-Libraries_51297f.pdf
The same as the previous help document, but is more updated and also contains the description of the A/D, PWM, I/O Port, etc. hardware libraries. This will probably be the most useful out of all the documents listed here.
The datasheet for the PIC18F4620 will also have to be consulted when using certain peripherals such as the A/D, PWM, etc.:
PIC18F2525-2620-4525-4620 Datasheet.pdf