Full documentation of all the aspects of the HPL2 Engine by
Specific information on Script Functions available in Game Specific - Amnesia
HPL2 comes with a great set of Tools to ease the development.
Tutorials for all tools, scripts and engine usage.
Information on Third Party Tools that can be used to create models, graphics and art for the game and what specific things you will need to setup to make it all work.
Additionally there are plenty of Resources available, including upgraded tools and script libraries to aid in development.