One of the lucky breaks in my early newspaper career was being assigned, in the Detroit bureau of the Wall Street Journal, a desk that was within earshot of Norman Pearlstine. This gave me the chance to listen as Mr. Pearlstine worked his sources, a labor for which he was famous. Once an automobile industry contact balked at providing Mr. Pearlstine production numbers, and Mr. Pearlstine told him that he would publish his own estimate — adding, “and I always estimate low.” That line was hung on the wall of the bureau, where over the years it would be shown to reporters aspiring to be like the man who went on to be the Wall Street Journal’s managing editor and the editor in chief of Time Inc.

The anecdote turns up on the opening page of Mr. Pearlstine’s new book, “Off the Record” (Farrar, Straus and Giroux, 304 pages, $25), in which one of journalism’s premier practitioners explores the nature of news sourcing — and the legal thicket it has become in the middle of the current war. His book grew out of the controversy that erupted when he decided that Time would bow to the Supreme Court of the United States and answer a subpoena that had been issued to Time and one of its reporters, Matthew Cooper, in the case that the special prosecutor, Patrick Fitzgerald, was pursuing against I. Lewis Libby. Mr. Pearlstine’s decision ignited an enormous hubbub. Denunciations were issued by the New York Times, important regional newspapers, and a number of individual reporters, even within Time itself.

But there were others who felt not only that Mr. Pearlstine made the right decision but that Time had a legal — even moral — obligation under the due process clause of the Constitution to provide the evidence it was holding. It was fine, even heroic, for Time to exhaust all legal remedies to protect its source. But once it had gone to the Supreme Court and lost, it seemed to many that the course was clear. On vacation at the time, I dropped Mr. Pearlstine a brief note to say that while he’d had many fine hours, this was surely one of them. In the book, he tells how his views evolved, starting with his days as a clerk in the law office of his father, Raymond Pearlstine, one of the great lawyers in Pennsylvania, up through the trial and conviction of Libby.

Mr. Pearlstine sketches the history of efforts by reporters to resist providing evidence. He writes of the case in which Judy Garland sued for defamation over a New York Herald Tribune story about her weight. Mr. Pearlstine quotes the trial judge as calling the reporter, whom he ordered to serve 10 days in jail for refusing to name her source, “the Joan of Arc of her profession.” Another touchstone in terms of case law is Branzburg v. Hayes, in which the Supreme Court dealt with four different disputes and, by a vote of 5–4, ruled, as Justice White put it for the majority: “The issue in these cases is whether requiring newsmen to appear and testify before state or federal grand juries abridges the freedom of speech and press guaranteed by the First Amendment. We hold that it does not.”

Branzburg was one of the precedents Mr. Fitzgerald used in moving against Time — and other journalistic institutions — in his pursuit of Libby. Mr. Pearlstine relates that his initial instinct was to stonewall the prosecutor, but that as he got into the subject, his views began to change. A lawyer as well as a newspaperman, Mr. Pearlstine tells the story with great fluency, including illuminating glimpses behind the scenes. One is a jocular e-email exchange between the special prosecutor, Mr. Fitzgerald, and Mr. Cooper’s lawyer about the fact that when Mr. Cooper got the call from Libby releasing him from confidentiality, Mr. Cooper was standing at home without his clothes on. The capacity for chuckling among opposing officers of the court is one of the bizarre aspects of lawyerdom.

Mr. Pearlstine emerges from all this advancing two broad reforms. One is in journalistic practice, taking greater care in offering confidentiality to sources in the first place. Mr. Pearlstine differentiates between “anonymous sources, whose identify we would pledge to keep out of our publications, and confidential sources, whose identity we would protect in the face of court orders and contempt citations.” A second reform he favors is a federal shield law that would give journalists a degree of protection in maintaining confidentiality of their sources. The case for the two approaches will rarely, if ever, be put better than Mr. Pearlstine has put it here.

By my lights, however, the big default in the Valerie-Plame-Lewis-Libby affair was not the decision of the courts to give a priority to due process in the face of the threat that subpoenas might represent to a press protected by the First Amendment. The big default was the decision of President Bush’s acting attorney general to name a special counsel in the first place. Such a prosecutor is, by the very nature of his assignment, immune to the normal sorting of priorities that would take place among government officials determining how to allocate their resources. There are few clearer lessons in recent decades than that no mortal, no matter how great his or her rectitude, seems to be able to resist the temptations that come with the unlimited power to prosecute. Hence an investigation was pressed long after Secretary Armitage had informed the Justice Department that he was the source of the leak of Valerie Plame’s name.

The ultimate danger is less to the press than to the presidency. The real culprit in the Plame-Libby case turned out to be the Central Intelligence Agency. Here was an arm of the executive branch of the government that was moving aggressively, in the midst of a war, against the civilian leadership of the executive branch. The episode has taught us how the presidency can be badly damaged by an out of control intelligence agency. This is more worrisome than the fight between the courts and the press. For the newspapers, magazines, broadcasters, and bloggers have always been a wily and resilient band. The press will always have the last word, and not only can the press, as one of our greatest editors once said, always estimate, but it can always estimate low.