Secondary sources address a topic “with at least one degree of separation”: they are not firsthand accounts of events or original creative works. They draw upon primary sources for “commentary, analysis, or critique,” and may also draw upon other secondary (or even tertiary) sources. The Library of Congress defines secondary sources as "accounts that retell, analyze, or interpret events, usually at a distance of time or place."
Your own paper will be a secondary source, combining evidence from primary sources with arguments from other secondary sources that you will respond to in your own argument. Generally, the secondary sources you will use for this paper will be scholarly sources.
Scholarly sources are generally written by scholars (who usually have a PhD or other advanced degree and are usually professors), primarily for an audience of other scholars in the same field. Because they address very specific topics and assume a high degree of familiarity with the field, they do not necessarily provide introductions to a subject that are suitable for a general audience. Scholarly sources make evidence-based arguments that build on previous scholarship; footnotes and other references are most easily spotted sign of this approach. Scholarly sources are generally published in peer-reviewed journals or by academic presses.
Book reviews that appear in peer-reviewed journals in many ways resemble scholarly articles, but usually should not be cited as sources. They are, however, very valuable to read in that they alert you to the existence of relevant books, tell you how the parts of that book fit together, and give you a sense of how that book fits into a larger conversation on that subject.