Assessing Secondary Sources
You've done your library work, assembling a pile of primary and secondary sources on your topic. Here are some tips for evaluating the secondary sources you've gathered, and figuring out which ones to include in your work:
Where did your secondary source come from? Different publications have different standards of reliability. Look for materials that have been peer reviewed. Before a book or article can be published by an academic publisher, it is sent for review by established experts in the field, who weigh its accuracy, its reliability, and the contribution it makes to the field of study; these reviewers also provide feedback to help the author improve the work. On-line resources that are not peer reviewed can be very useful for helping you find materials and information, but they are often not peer reviewed. (Depending on your topic, such resources--blogs and other on-line essays--can, however, often be considered as primary sources!)
When was the source written? New discoveries are being made in musicology all the time, and the conversation about music is always changing. Therefore, you want your secondary sources to be as up-to-date as possible. As a general rule, try to use secondary sources written within the last 20 years. Since these all use citations (footnotes, endnotes, bibliography) they will point you toward lots of other relevant literature produced earlier, so you'll be able to follow those leads, too.