MONGOOSE R.I.P. By William F. Buckley Jr. Random House. 322 pp. $17.95

MORTAL GAMES By Pierre Salinger and Leonard Gross Doubleday. 319 pp. $17.95

THREE STREAMS feed the flood of spy fiction. From one of them, less turbulent than it once was, tumble the extravagant fantasies of James Bond and his clones. These fairy tales are marked by absurd plots, brainless tradecraft, fantastic gadgets -- the more astonishing because they always work -- and the wholesale, carefree slaughter of the enemy. The handsome heroes are padded with unlimited funds, never suffer a tremor of doubt and always see their endeavors crowned with total success.

Curiously, although James Bond is still the most universally recognized name in spy fiction, few readers notice that he rarely acts either as a spy or as an intelligence officer. With the notable exception of From Russia With Love, in which he makes off with a Soviet cipher machine -- and nearly dies as a result -- Bond usually comes across as a kind of international supercop. When he is not on a holiday, 007 busies himself thwarting deranged geniuses seeking to loot Fort Knox, or plotting to hold the world hostage to some doomsday device. Bond never troubles to conceal his identity and rarely stoops to anything that smacks even slightly of the practices of the real secret world.

Groping their way along another stream bed, and through what sometimes seems to be impenetrable gloom, come the followers of the realist school, created almost singlehandedly by John le Carre'. Meant to outdo the original, their heroes have less humor, are more angst-ridden and just as grubby as some of Smiley's crowd. Fortunately their foes, though thoroughly professional, are also no more than life-size. Even at the moment of bitter victory, the glum winners pause to contemplate the human face of their vanquished adversary.

If the reader is willing to discount a measure of the bleak atmospherics -- even in spy country it doesn't rain every night -- and to cast out a bit of the treachery and base motives usually attributed to the protagonists' chiefs -- some of these books will reflect a bit of the real thing.

Fortunately for many readers there is a third and livelier stream welling into the headwaters of espionage fiction. The plots of the best of these books are often embroidered around incidents and persons well known to every newspaper reader. The heroes, though good-looking and sometimes handsome, are sufficiently vulnerable to seem plausible, none takes himself too seriously, and a few are blessed with a sense of humor. Even the villains are appropriately complex and moved by recognizable motives.

In Mongoose R.I.P., William F. Buckley Jr., a master of this third genre, has put Blackford Oakes, of Yale and the CIA, back to work for the eighth time. It is January 1963, and the attorney general has just informed Oakes' boss and intelligence mentor that he is to undertake the assassination of Fidel Castro.

In the next several pages the reader meets a Cuban porn star and favorite of Castro; learns that Rolando Cubela, a Cuban patriot, is for the best of reasons also determined to kill Castro; eavesdrops on a long and convincing internal monologue of President Kennedy; spends an hour or so in Khrushchev's office; and sits in on an equally plausible meeting in which Castro, his brother Raul and assorted henchmen are briefed on the CIA organization in Miami. In a deft mix of fact and fiction, each of these characters -- real or imagined -- springs vividly to life.

THIS, ALAS, is more than can be said for Blackford's beloved Sally Partridge, a really boring product of Vassar and Yale. As one of the point men in the battle against world communism, Oakes must remember that he has too much on his plate to allow himself -- and his readers -- to be distracted by a liberal bluestocking who has welcomed the opportunity to lecture Mexican graduate students on Jane Austen.

Along with providing truly surprising twists in his plot -- the more unforeseen becasue we know Castro is still in office -- Buckley takes time to thump a few of his favorite devils, and proffer his usual political judgments and asides.

Mongoose R. I. P. is entertainment of the first order.

IN SHARP CONTRAST to Buckley's adroit mix of fiction and fact, authors Pierre Salinger and Leonard Gross' new novel is all fiction. Mortal Games begins a few weeks after the close of their earlier novel, The Dossier.

On the heels of two sensational news stories, Andre' Kohl, the senior European correspondent for an American television network, has decided to give up his lush Paris existence, marry the daughter of the acting director of the CIA, and get back to music and a more contemplative life.

Things start going wrong at a lavish Paris dinner party celebrating Kohl's retirement. Before Kohl can give his valedictory, gunmen spray the podium with machine pistols. The guests -- "Tout Paris", all 200 of them -- scramble. Kohl survives and is spirited to Washington by the CIA. Mercifully this happens after Kohl has put away his share of Chateau Petrus Pomerol 1966 which we are told goes for 9,300 francs a bottle, or about $200 (old dollars) a noggin, service and tax non compris.

In Washington, Kohl learns that because of his broadcast exposing a sinister network operating within the CIA -- but unbeknownst to the agency -- he is a candidate for murder. Along with carrying out their own grim foreign policy, the renegades are sponsoring an ex-Nazi paramilitary organization intended to throttle liberal movements in Latin America. Despite elaborate precautions, Kohl and his bride are gunned down on their honeymoon. Both survive, but on the advice of the CIA, Kohl agree to play dead. No one, including his wife, is to know that he is alive and preparing to go after the renegades.

After four months of strenuous CIA retooling, Kohl is born again. Plastic surgery changes his face; voice teachers supply a new timbre and accent; and a trainer fits him out with a brand-new body. Always a good soldier, Kohl even exchanges his bilingual French for a clumsy and accented American version. When it all proves to work so well that neither Kohl's "widow" nor his old television chums recognize him, the hunt is on. It takes Kohl back to Paris -- wealthy readers planning a trip should note the restaurant tips -- and on to Moscow, and points east. Before it is over, the reborn Kohl chairs a meeting between the director of central intelligence and the KGB chairman.

Accept a premise or two, and sit back with a lively story enriched with inside glimpses of the Parisian fast track.

William Hood, a veteran of the OSS and the CIA, is the author of "Mole," "Spy Wednesday," and the forthcoming "Cry Spy."