WE DON'T KNOW for sure who put the bombs on TWA 840 on April 2, or in La Belle disco in Berlin on April 5. But the best guess is that the trail leads back to Libya -- and we know who made the explosives and timers, who sent them to Libya and who trained the Libyans to use them.

Those bombs were as American as apple pie. They were made of C-4, an American plastic explosive of devastating power, that can be spread flat, like a sheet of paper, molded to look like a pen, a pack of cigarettes or pocketbook -- and slipped easily under an airplane seat. It is perfectly safe to handle -- until detonated. Timers can be made as small as a pen cap, and set to explode hours or days ahead.

The C-4 was made in Louisiana, Texas and New York. An American explosives dealer, Jerome Brewer, sold Qaddafi 21 tons of it. CIA experts sold him thousands of timers (he ordered 500,000), and off-duty Green Berets trained the Libyans in their use. It was American greed, not Soviet malevolence that made Muammar Qaddafi the best equipped terrorist in history.

The man who orchestrated the sale to Libya of the materials and expertise to blow up American airliners, and bars in Berlin, Paris and London, was Edwin P. Wilson, formerly of the CIA and naval intelligence. Manhunt describes how he was caught.

The story has been told before, in The Death Merchant by Joseph Goulden and in scores of newspaper and magazine pieces. Maas adds the Barcella factor. Lawrence Barcella was an ambitious and publicity-concious U.S. attorney in Washington. He brought the case against Wilson, overriding the institutional cover-up that protected him, getting him indicted and then luring him home from safety in Libya.

Playing up Barcella's part isn't enough to justify a book. His role has been thoroughly reported already, and we don't need to be told that he worked long hours and missed a lot of sleep.

But there's a different justification: the extreme importance of the story itself. Maas tells it clearly and well.

It's not only guilty individuals, it's institutions, most notably the CIA. Senior officials knew Wilson was dealing with Libya -- and took his money shamelessly. When they left the agency, they went into business with him, even after he had been indicted. According to Maas, one of them, Theodore Shackley, hoped to be appointed director of the CIA if Gerald Ford had won the 1976 election.

Instead, the job went to Admiral Stansfield Turner, who sacked many of the old guard and has been roundly abused ever since for doing so.

Wilson had a remarkable gift for using his official positions to make money. He was paid less than $30,000 a year, and became a millionaire. He set up "proprietories" for the agency, companies ostensibly engaged in legitimate trading that were intended to serve the agency's secret purposes. Wilson used his agency contacts to bring business to his companies -- and kept the profit. He spent lavishly on a palatial estate in Upperville, lived like a Mellon and put dozens of generals and current and former agency and other officials on his payroll. The companies thrived, and when he was finally sacked by Inman he kept the companies -- and the contacts.

In the later '70s, Wilson slipped over from ordinary corruption to treason. The instrument was another crook, Frank Terpil, who was also trading on his former connections with the CIA, and who had made contact with Qaddafi. These were the years of the great oil boom, and Qaddafi wanted to make himself a world power. Wilson supplied him with contraband from the United States, spare parts for his C-130 cargo planes, uniforms and other military supplies -- and the C-4 and timers. The controls meant to stop smuggling arms and explosives failed completely. Wilson also recruited former Green Berets to set up a terrorism school in Tripoli.

A DISGRUNTLED ex-employe, Kevin Mulcahy, blew the whistle on him -- but he was protected by his old colleagues who were also his partners or employes, and it took several years before official Washington finally understood how serious the situation was. Eventually, Barcella got Wilson indicted for smuggling and for attempted murder. He had tried to recruit Cuban exiles to murder a Qaddafi opponent in Egypt, and did recruit a Green Beret to attempt to murder another opponent in Colorado. Wilson took refuge in Tripoli -- and was eventually lured out by a free-lance con artist, who sold Wilson worthless land in Florida and persuaded him he would be safe in the Dominican Republic. Barcella had the marshalls waiting for him and he is now serving a long prison term.

Goulden's book included many of the tall tales of Frank Terpil and Kevin Mulcahy. Maas skips the romantic details (though the anecdotes he can corroborate are hair-raising enough). He sticks to his narrative and this emphasizes the astonishing negligence of the CIA in its dealings with Wilson. It wasn't just the CIA. Senators and congressmen and their aides, and senior Pentagon officials were Wilson's guests on his palatial estate in Virginia, yet none of them questioned where he was getting the money.

This was immediately after the Watergate scandals. It's an unsettling reminder of how easily and far corruption can spread. Some of the people Wilson tried to hire (including the Cubans) turned him down. But Wilson bought dozens of others, skilled, experienced people, like so many hookers. In the 1930s and '40s the traitors were idealists. Now they are just crooks.