A secondary source discusses, analyzes, and interprets a topic. Typically, in history, a secondary source will cite both primary sources and other secondary sources used to develop the author's argument. A historian's work, either a book or journal article, typically is a secondary source. Your senior thesis will be a secondary work.
You will want to use secondary sources for a couple of key reasons: 1) to gain a clear understanding of the events, time, people, places for a topic, and 2) to use the work cited to identify key primary sources.
Secondary sources are contextual. So, for instance, a newspaper article may be primary source for one topic (an first hand account from the time being studied) OR if could be a secondary source (discussion of an event that occurred in the past).
The databases listed here are places to start when searching for secondary sources.
WorldCat Discovery is the public version of WorldCat at https://www.worldcat.org. WorldCat is linked in Google Books, Goodreads, and more.
The Blume Library also subscribes to WorldCat FirstSearch. FirstSearch offers precision searching where you can input many types of limits. For instance, you can sort your results by number of libraries that own the item which can impact the ease or challenge of receiving an interlibrary loan.