Angela is a paralegal who works for Paul, an attorney representing the plaintiff in a commercial-litigation case. Paul wants to obtain copies of certain documents from Ruf-n-Tuf Chainsaw Company, a third party to the lawsuit. Paul is particularly interested in correspondence between the defendant and Ruf-n-Tuf. Paul asks Angela what method, under the discovery rules, is the best method for this purpose. Of the four main types of discovery under the federal rules, which of the following is the only method that can be used?