This is a rather vaguely-worded question. What are these topics? What is the research for? What do you mean by a second-hand source?

Assuming you mean writings by historians commenting on primary material, I would say as a rule of thumb to go for books and articles over websites (at least if it’s a website, it should have citations, i.e. references to the material it draws from). Google Scholar can be a good tool for this; look for resources that are recent (they will cover relevant previous important works) and have a good number of citations. Google the author and judge whether they are a reputable source.

A word of caution - historians will have their own theories and explanations for historical events, even if their writing appears as ‘narrative’.