Are analyses, interpretations, and evaluations of primary sources. They are usually aim to give reflection or analysis. They’re classified as index type, survey type and reference type.
INDEX TYPE Typically, these are found as one or more individual volumes at the end of a set. Some of the examples are the bibliography, index, indexing periodicals and abstracting periodicals.
SURVEY TYPE It involves the product of examination or the description of someone or something. Examples are reviews, treatises and monographs.
REFERENCE TYPE Consist of materials collected from other’s works. Such as encyclopedia, dictionary, handbook, manual and critical tables.