Which philosophers write about subjectivity? What are some primary sources for philosophical theories of subjectivity?
For a general overview of this very interesting subtopic in philsophy of mind:
What is it like to be a bat? by Thomas Nagel
Could love be like a heatwave? by Janet Levin
Knowing one's own mind by Donald Davidson
Individualism and self-knowledge by Tyler Burge
Anti-individualism and privileged access by Michael McKinsey
For further reading, you might considering checking out some of these as recommended by my philosophy of mind anthology:
Depending on what one means by "subjectivity", the answer is "pretty much all of them."
The question is so broad as to make an intelligent answer difficult.
That being said: if we want to point out a few of the more radical views, Nietzsche's dictum that there are no facts, only interpretations is certainly a noteworthy milestone, as is Wittgenstein's beetle-box thought experiment.