Members of the Secret Service are immediately apparent at events that provide a glimpse of a president: They are the ones staring at the audience, rather than the attraction.

According to Dennis V.N. McCarthy, who retired last year after 20 years with the service, much of it on the White House detail, agents always see the same thing: the words "There he is" forming on the lips of enthralled observers.

On March 30, 1981, while watching such a scene as President Reagan emerged from the Washington Hilton Hotel, McCarthy heard a "pop no louder than a firecracker." Almost instantly he recognized the sound as that of a gun and saw among the television cameras aimed at the president two hands gripping a pistol.

Most of us, by instinct, would dive for cover. McCarthy, by training, dove toward the gunman. He came down on the back of John W. Hinckley Jr. just as Hinckley fired his last bullet. As his weight caused Hinckley to collapse, McCarthy heard a "rapid click, click, click" as the would-be assassin continued to pull the trigger.

President Reagan was wounded. So were James Brady, his press secretary, a fellow Secret Service agent Timothy McCarthy (no relation) and Washington police officer Thomas K. Delahanty. Hinckley, taken into custody by Dennis McCarthy and others, was later institutionalized as insane.

Secret Service agents, according to McCarthy, pledge their willingness to exchange their lives for that of the president. By their code, anything less would be cowardice. None of them can know, however, whether he would fulfill the obligation. McCarthy did not know, and now he does: "I had met my dragon on that gray spring afternoon, and I had conquered him."

He shortly met other dragons. After the shooting McCarthy went to a hospital for X-rays of his neck, strained when he tackled the gunman. His wife drove him home from the hospital.

When he got home, McCarthy found a message on his telephone answering machine. It was a man's voice, asking his wife if he was the "McCarthy" who had been shot. McCarthy was sure the inquiry was being made on behalf of his wife's lover. He confronted the caller the next morning, and "When I hung up, I knew my marriage was finished."

Despite the public and personal trauma, McCarthy continued to work 12-hour shifts in the days following the shooting but soon found himself unable to concentrate. At first, he blamed himself for not reacting more quickly to the sound of gunfire -- until videotapes proved he had leaped at Hinckley by the time of his second shot. After a period of severe mental stress, he was assigned to a State Department job until his retirement last year.

"Protecting the President" begins with the Reagan shooting, a taut account with its unexpected personal aftermath. The rest of McCarthy's story proves harder to tell, especially as presented in a haphazard series of recollections of a Secret Service career that includes chasing counterfeiters in New Orleans and protecting four administrations in the White House.

McCarthy has no shortage of opinions about the elected and appointed cadres he has served. H.R. Haldeman, a "power-hungry egoist," remains stuck in McCarthy's craw, accused of a five-year effort to turn the Secret Service "into his own private police force." President Johnson was a "royal pain" who treated his protectors like ranch hands, and Henry Kissinger was a "real pain" who once asked McCarthy to order agents to accompany him swimming because of reports of sharks. Secret Service protection in that case was deemed to stop at the water's edge.

McCarthy is candid to a fault in describing some of the "pressures" on Secret Service marriages. As he puts it, "However much extramarital activity there is on the part of some of the Secret Service agents, the job is a lot more work than fun." One of his own affairs began in Cairo, where he met an Austrian in a bikini. She followed him to Rome, and later to Paris and still later to Washington. McCarthy tries to recount his own escapades -- and their terminating effect on his two marriages -- with objectivity. The effect of such confessions by a public hero is curiously unsettling. The terrain is perhaps better left to novelists, sexual autobiography being more palatable at a slight remove.

"Protecting the President" has little to say about the Secret Service as an institution. The selection and training of agents and details of their methods are entirely missing, when they might have helped us understand how McCarthy was able to do his job so well. This is not an issue of national security but of authorial laziness. Much more could be told without compromising the mission of the agency, but it is always easier to yield to the secrets-keepers.

The value is not in the book but in the man. Dennis McCarthy, in a moment, took an action which justified himself to himself. It's worth something to be reminded that action has the capacity to justify.