1. Books: This may be electronical or physical. They will always be my number one choice, but electronic ones are more convinient to use.
2. Journals: It is quite helpful to read new investigations, conclusions and tendency topics that will complement your information.
3. People: I think that interviews, nice stories and observation are fundamental keys to make conclusion to any research.
In my personal experience I have never liked newspapers, information sometime's not accurate or true.