Apropos of the latest scandal, here's an interesting report from the Congressional Research Service on Congressional Investigations: Subpoenas and Contempt Power. It has analysis of the law and lots of historical case studies.

Here's their conclusion:

Committee subpoenas and contempt citations have been effective instruments for gaining access to executive branch documents that are initially withheld. The pressure that builds from these two techniques generally results in the Administration offering new accommodations to satisfy legislative needs. Although both branches at times seek assistance from the courts, the general message from federal judges is that an agreement hammered out between the two branches is better than a directive handed down by a court.

The executive-legislative conflicts described in this report offer several lessons about access to information. Congress has as much right to agency documents for oversight purposes as it does for legislation. Executive claims of “deliberative process,” “enforcement sensitive,” “ongoing investigation,” or “foreign policy considerations” have not been, in themselves, adequate grounds for keeping documents from Congress. On the issue of withholding information from Congress, there are often sharp differences within an Administration, especially between the Justice Department and the agencies.

Further, these case studies show that statutory language that authorizes withholding information from the public is not a legitimate reason for withholding information from Congress. Sharing sensitive information with congressional committees is not the same as sharing information with the public. Courts assume that congressional committees will exercise their powers responsibly. Legislative committees have demonstrated that they have reliable procedures for protecting confidentiality. Finally, congressional capacity to subpoena agency documents from private organizations is not an adequate substitute for receiving them directly from the agency.