Secondary source material serves several purposes for the historian. These sources provide:
- background overviews of a topic
- scholarly interpretations of primary source evidence
- citations to primary source materials
- bibliographies leading to other secondary sources
Ask yourself if what you have is biased in any way. How old is the publication? Has new evidence or historiographic methods appeared since the publication of the item at hand? What question does the text ask and is it answered? What questions remain?
This means you are going to deep dive into books and scholarly journals. Expect to find differing viewpoints and interpretations and be sure to bring these into your own work. Not that you necessarily agree with the arguments you find but you need to acknowledge the disagreements and provide evidence to support your interpretation. This is entering the scholarly discussion and part of being a historian.
In the case of books, it might be helpful to look at book reviews written by history scholars (not the promotional blurbs so popular with publishers.) scholarly book reviews will point out strengths and weaknesses in the research methodology and resources. This can be very helpful as you become acquainted with key contributors, current arguments, and sources on your historical topic.