In history, secondary sources are most often scholarly books, articles, and dissertations that interpret and analyze the past. In other words, secondary sources are what your history professors (and scholars like them) write.

Historical secondary sources share many of these attributes:

Written by a scholar
Books are often published by an academic press while articles are published in journals rather than popular magazines
They are peer-reviewed (aka refereed) -- reviewed for quality, accuracy, and originality by other scholars in the field before publication
Evidence-based through the use of primary sources
Include extensive footnotes/endnotes and bibliography (this makes it ideal for identifying possible primary sources)
Written for a specialist audience rather than the general public

Secondary sources play an essential role in historical research by:

Providing context for a topic. For example, if you were researching racial covenants in Seattle neighborhoods, secondary sources could provide you with a history of African Americans in Seattle, history of residential segregation in the United States, and a legal history of municipal zoning laws.
Providing the scholarly conversation surrounding the topic. New scholarship is informed by past scholarship on a subject. It builds on earlier scholarship, sometimes incrementally, other times in a more dramatic shift.
Providing leads to primary sources. Footnotes are important in historical research because they serve as the evidence for a historian’s argument. Footnotes also provide us a glimpse into the scholarly conversation or historiography surrounding a topic.

Also keep in mind that historical scholarship is not static, rather it evolves and reflects shifting areas of interest due to changes in the demographic make-up of historians, as well as, current political and social issues, and new theoretical frameworks. As historian Boris V. Ananich wrote in "The Historian and the Source: Problems of Reliability and Ethics":

"A working biography of a historian, his or her publications, helps readers to learn not only about the author’s personal professional qualities, but also about the time in which he or she lived and the priorities he and his contemporaries may have chosen in studying the past."

Optional Activities:

Want more info on peer review? Watch: Peer Review in Three Minutes (KU Libraries).
Read: Prof. Warren's dissertation, book, and article shown in the banner.
Learn about strategic reading via this module in the UW Libraries Undergraduate Research Tutorial.
Read: Sandage, Scott A. “How to Read a Book: The X-Ray Method for Achieving a Sustainable ‘Book-Life Balance.’” Commonplace.