Primary sources in music are a direct representation of the work of the composer or author. This is different than secondary sources—books and journal articles—which often analyze primary sources. And tertiary sources—encyclopedias and dictionaries—are often research done on secondary and primary sources.
A primary source is an original work. Types of primary sources include:
An example of a primary source for John Cage is Letters by John Cage.
Secondary sources are one step removed from primary sources. Often they are an analysis, critique, or interpretation of a primary source.
Types of secondary sources include:
An example of a secondary source for John Cage is a website about Cage's writings.
Tertiary sources are often a combination and examination of primary and secondary sources. Examples include:
An example of a tertiary source about John Cage is an article about him in an encyclopedia on experimental music.