I am reading The Indians of Texas by University of Texas professor W.W. Newcomb, and while it has some interesting information, there's some persistent terminology going on. He uses words like "savages," "barbaric," "primitive" and other shudder-inducing terms. He defends them in one breath, but seemingly demeans them the next. How do you interpret sources and authors who seem to a product of their times?
The same way I interpret ancient sources and authors - by attention to their context and as historical documents.
To put it another way, of course Newcomb is a product of his time. How could he be otherwise? In the same way, whatever historical research you produce will be a product of your time, and in 40 years someone might read your papers and think, "Gee /u/octopodesrex really goes to town with their biases. How can we take their writing seriously?"
Understanding the pervasity of subjectivity is a pre-requisite to dealing with it. And realising that one is also subjective and that 'dealing with it' doesn't mean producing idealised "objective" work is also essential. Nor does the realisation of inherent subjectivity imply philosophical relativism. Just the reality that all history is subjective, and all history writing is the production of documents that are themselves historically conditioned, no more or less than the object of that piece of history writing.
Remember, as much as Newcomb is a secondary source on Texan Indians, he is a primary source on "1950s-50s historical method as practiced in Texas in regards to Native Americans."