Topic areas of continuing and rapid development, such as the sciences, demand more current information. On the other hand, topics in the humanities often require material that was written many years ago. Further editions indicate a source has been revised and updated to reflect changes in knowledge or to include omissions. Also, many printings or editions may indicate that the work has become a standard source in the area and is reliable. If the source is published by a university press, it is likely to be scholarly.
Primary sources are the raw material of the research process.
Secondary sources are based on primary sources. For example, if you were researching the Oregon Trail, a diary kept by one of the pioneers is a primary source, while a book using that diary is a secondary source. An efficient search of the World Wide Web will produce hits that target your subject interest very accurately. This is helpful if you want to see what might be available in a general category. Consider the qualifications, other works, and organizational affiliation of the author.