The types of sources you will be looking for to write a strong historiographic essay include monographs (books), journal articles, and chapters in edited books. These can be scholarly, meaning they are written and reviewed by experts in the field, written for students or other scholars, and with rigorous attention to citing sources; or they can be popular, meaning written for a general audience and without many (or any) references.
Here's a definition according to the Oxford English Dictionary (a classic reference resource):
/ˈmänəˌɡraf/ noun
a detailed written study of a single specialized subject or an aspect of it. For example: "a series of monographs on music in late medieval and Renaissance cities."
A monograph is original scholarship that presents critical analysis based on extensive research of primary sources.
NOTE: Scholars frequently use the word "book" and "monograph" interchangeably, but technically, a book can also be a collection of collected essays, or a textbook, or a novel. The word "monograph" is generally used when speaking about a scholarly work on a single subject.
Look for several things to determine if a book is a scholarly monograph:
{Adapted from: California State University Northridge Libraries, Research Strategies}
Scholarly articles are written by scholar, researchers, or experts in a field in order to share the results of their original primary research or analysis with other scholars, researchers and students. They are shorter than books, but are generally 8-20 pages in length, but sometimes longer.
Academic journals are published on a regular basis (2 or 4 times a year, usually) and each issue includes many different articles by different authors, like a magazine, but for scholars!
Here are some qualities that set scholarly articles found in journals apart from "popular" articles such as are found in newspapers, magazines, etc.
{adapted from: University of Wisconsin-Madison, Identifying Scholarly Articles}
An edited book is a collection of essays/articles (or chapters) by various authors that are selected by one or more editors and collected into a single published book.
Edited books are different than scholarly monographs in that every chapter in a monograph is by the same author.
Encyclopedias, almanacs, handbooks, guides, and style manuals are all examples of reference sources.
Reference sources are valuable in the beginning phases of the research process. They can help you find quick, reliable facts and background information. They can provide a broad overview of a topic, supply keywords and concepts that are useful in searching for books and articles, and may list important scholarly works in the field to consult.
Primary Sources are immediate, first-hand accounts of a topic, from people who had a direct connection with it. Primary sources can include: