In view of the current conflict between Congress and the President, it might be useful to outline existing law on executive privilege, and thus to provide a kind of primer (a tentative, preliminary, and incomplete one to be sure). The following does not focus on or attempt to resolve the current controversy.
Nor does it trace the practices of Congress and the executive branch over time. One of the largest lessons is that much of the law remains unsettled. The Supreme Court has not said a great deal; the court of appeals for the District of Columbia Circuit has said more; but it is not at all clear that a majority of the Supreme Court would agree with what the lower court has said. Here is (the bulk of) the law as it now stands.