What are good resources on causes of private aircraft crashes?
The AOPA's Air Safety Institute has quite a few resources that I like to review from time to time, including some great videos with commentary on exceptional cases.

AOPA ASI Accident Analysis Page
AOPA Accident Case Studies
AOPA Real Pilot Stories

I do exactly what you are doing, during training and still after, but I'd like to also say that you shouldn't narrow yourself to GA crashes, a lot of what happens commercially can be applied to GA, and usually with commercial accidents there is a lot more data and analysis available to work with. In that regard, I like to watch shows on Youtube like "Air Crash Investigation", or on TV like "Why Planes Crash" (some available on Youtube)

AOPA also has a lot of resources to view NTSB reports on individual crashes where you can search by certain criteria like aircraft type, states, type of flight, or even some specific keywords like "Vacuum Failure".

Just don't limit yourself to looking at things like "Pilot Error", pilot error is a broad category of crashes that is usually used to describe the pilot doing something wrong in the air, but a lot of crashes are "pilot error" in that the pilot could have broke the accident chain much earlier in the flight. Read as many as you can, regardless of the cause. For example an accident where the aircraft breaks up in flight may not be attributed to pilot error, but its good to know what lead to the break up and what could have been prevented.