Sometimes you may come across a potentially interesting source in another source. For example, a book is cited in an article you are reading. In this case, the book is considered the original (primary) source, while the journal article citing the book is the secondary source.
Avoid citing secondary sources when possible. Generally speaking, you should only cite works that you have actually read. If you are unable to get hold of the original work, for instance if the work is an out-of-print book, an old print-only report, or available only in a language that you do not
Suppose Smith's work is cited in Jones, and you did not read Smith. In your text, mention the primary source (Smith) and provide a citation for the secondary source (Jones). Use the phrase “as cited in.”
Please note If the year of the primary source is unknown, omit it from the in-text citation. Do not use the abbreviation "n.d." (for "no date").