Everything you need is available on line. Get a C reference - google turns up many - "C reference manual". There is a microchip C18 user's guide which discusses their extensions to the language.
Ah.... I get it, I was looking for a reference for explicitly C18. I was searching to specifically.
I found the Microchip C18 users guide and it's reference to the extensions to the language last week and made some notes on the extensions and their usage.