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You will usually be expected to find scholarly articles in academic journals -- specifically, in what are called "refereed" (or "peer reviewed") journals. This means that the articles have been reviewed ("refereed") by experts in the field, and that these experts have judged the articles to be worthy of publication.
In a database such as Historical Abstracts, most of the results you get will be peer-reviewed articles. However, you can be certain of whether or not you are finding refereed articles by selecting "Peer Review" from the list of search refinements that is available on the left sidebar of every results page.
Books: Balance basic keyword searches in the library catalog with advanced searches that permit searching by the "subject headings"
Books can be secondary or primary resources.
A scholarly monograph is a book with one (mono) author, written for an academic (scholarly) audience.
Note: It can be helpful to look for book reviews for scholarly monographs. Academic book reviews will place that book in disciplinary context.
Search these resources for books.