Rules questions


I have some rules questions that I couldn't find in the FAQ:
Page 66 - Attentive: It references Investigate, what is that?
Page 69 - Diehard: What is considered 0 HPs, 20 or 25 pips on the lifebar?
Page 72 - Improved Damage Threshold: This feat references Massive
Damage Save. But that save throw is not explained or mentioned
anywhere else.
Light Weapon: There is nowhere that explains what a light weapon is.
Is size small considered light?
Dave M