XBMC Quick Start Guide: http://xbmc.org/wiki/?title=XBMC_Quick_Start_Guide
The official XBMC on-line Wiki documentation: http://xbmc.org/wiki/?title=XBMC_Online_Manual
The Wikipedia entry (the English version): http://en.wikipedia.org/wiki/XBMC
The official on-line forums (support, how-to, FAQ, etc): http://xbmc.org/forum/