There's Yvain's Limits of Introspection post, which links to a paper on the scientific study of what we can access introspectively. Yvain isn't exaggarating when he calls it a "classic": Google Scholar lists over 6000 cites.

You may also wish to take a look at the papers citing it to see if there's anything more recent that's relevant, though from a brief glance it looked like the top results were about something else than introspection entirely. Still, there are probably more relevant results buried there somewhere.