Searches usually go from the general to the specific. You might not actually do it that way, but you should be aware of it. If you are doing a very thorough search, look at all the different media available to you. Journal articles usually cover a small idea in a concise manner. Books may cover the basics that articles omit, but they also have the space to go into greater depth.
Dissertations are usually narrow but thorough.
Speeches and papers presented at conferences are current and fairly short, but are often overlooked by researchers. The Web can be the most current of all, but may not be reliable.