Should you begin with a primary document or author or read the critical secondary literature first?
Might not be a useful question to ask because:
If you need to make a decision:
Alternatively, you should just follow your own instinct and interests provided that you keep good notes that are well-linked.
(104){ How to Write a Thesis Eco, Umberto. How to Write a Thesis. Cambridge, Massachusetts: MIT Press, 1977.