if you are using C18 from microchip then when its installed goto
C:\mcc18\doc
there are a few documents there with lots of info:
hlpC18Lib.chm - "This section gives an overview of the MPLAB C18 library files and precompiled object files that can be included in an application"
hlpC18ug.chm - "This document discusses the technical details of the MPLAB C18 compiler. This document will explain all functionality of the MPLAB C18 compiler"
hlpPIC18ConfigSet.chm - "This document lists the configuration settings available for each of the PIC18 devices for use with MPLAB® C18's #pragma config directive and MPASM™ assembler's CONFIG directive. "
PIC18F Peripheral Library Help Document.chm - "The PIC18 MCU Peripheral Library provides a set of functions for setting up and controlling the operation of all the peripheral modules available in the PIC18 devices, as well as functions for interfacing with an external LCD, Software implemented communication modules. "