THE SISTERS. By Robert Littell. Bantam. 312 pp. $16.95; THE CROCUS LIST. By Gavin Lyall. Viking. 288 pp. $15.95.

THESE excellent thrillers -- The Sisters by Robert Littell and The Crocus List by Gavin Lyall -- have more in common than coincidence of publication. The inciting incident of one and the climax of the other are assassination attempts involving U.S. presidents. And in both Western intelligence operatives try to frame the KGB.

Robert Littell's seven novels are as good as thriller writing gets. Since the publication of The Defection of A.J. Lewinter in 1973, he has shown a talent for believable characterization, zany humor and deft plotting. Like the characters of Ross Thomas, Littell's toughest competitor in this racket, his people are as apt to get hit by a pie as by a fist and usually wind up getting pasted by both.

"The Sisters," a team of seasoned CIA operatives named Francis and Carroll, "were minor legends in the Company. Somewhere along the line one of the CIA's army of PhD's who majored in African dialects and minored in Whitman had dubbed them 'The sisters Death and Night.' The name stuck. If you mentioned the Sisters in an interoffice memo, and capitalized the S, almost everyone tucked away in the Company's cradle-to-grave complex knew whom you were talking about. But only a handful with 'eyes-only' authorization in their dossiers had an inkling of what they actually did for a living.

"What they did was plot.

"And what they were plotting on that perfect August day was a perfect crime."

The Sisters have figured out a way to awaken a "sleeping" Russian operative hidden in the U.S. At their command, he will be activated and ready to do anything, and anything he does will be blamed on the RussWith an almost innocent enthusiasm and the vaguest do-what-you-have-to-do-type authorization from the director, the Sisters launch the Sleeper. In response, both the KGB and the CIA set up damage control operations to clean- up the anticipated mess -- neither side knows who the Sisters have targeted -- and try to get an edge on the opposition.

The novel might as well have been called "The Potter" after its central character, Feliks Arkantevich Turov, once the novator, the "man in charge" of the KGB sleeper school. Living in forced retirement and, though only 57, feeling as if he were "tied up to the pier of old age," Turov spends his days in an attic workshop throwing clay. Squeezed into betraying his last surviving sleeper (and best friend), the Potter decides to take himself out of the game. His dangerous escape from the Soviet Union and desperate chase across America to save his proteg,e are the backbone of the novel.

Littell surrounds his main characters with a memorable supporting cast: the Potter's acquisitive sexpot wife Svetochka; the Sleeper, with his kinky sexual habits; his two live-in girlfriends; the bland, ruthless apparatchiks running clean-up and damage control on both sides, especially Appleyard and Ourcq, the freelance Canadian "sweepers" -- one a foul- mouthed Tweedledum, the other a Tweedledee whose repertoire of oral effects learned during his earlier career as a sound man on radio soap operas includes the noise made by the setting sun -- hired by the Russians to stop the Potter from reaching the Sleeper.

In almost any narrative there are holes. Littell, like any good storyteller, draws attention away from his by jumping up and down and pointing at them. Remember how in Casablanca so much is made of the "letters of transit" that you are dazzled into forgetting how completely ridiculous they are? Littell has similar knack for making the reader believe that occasional lapses in story logic make perfect sense. Later you may scratch your head and wonder how so-and-so could have known such-and-such but by then it doesn't matter.

The Crocus List is Gavin Lyall's ninth book, his third starring British Army Major Harry Maxim. In this one, Maxim is heading up a security unit guarding a U.S. president on a visit to London. When the presidential party is shot at, Maxim pursues and kills the assailant. In the subsequent official coverup (bureaucracies seem to loathe the idea of conspiracies), a scapegoated Maxim must discover the truth if he is to clear his record.

The Crocus List is as typically British as The Sisters is American. The plot is phenomenally complex. Expectations are set on their heads at nearly every turn of the page. The humor is at once subtle and biting. The delicate ironies of class, absent from considerations of American culture, not only form the backdrop of the story but often motivate the characters to action or inaction. The politics are also trickier. Although in both books the conspirators are attempting to stiffen their country's opposition to a national enemy, in The Crocus List the process of influencing foreign and defense policies is seen as more complex than simply blowing away a spineless chief executive.

Though I recommend The Crocus List heartily, I should say that the early pages are slow going. Major Maxim is a little too James Bondish to be true and the author expends several chapters filling in personal background of the kind series readers seem to crave but which doesn't advance the story. And there is an unnecessarily detailed exposition of British defense strategy which though relevant is dry enough to send you to the Land of Nod. It isn't until the appearance of Maxim's friend George Harbinger, a timid -- and tenacious -- upperclass alcoholic, dead-ended in the Legal Corps after a stay at No. 10 Downing during a previous administration, that things come alive. George, frightened, overweight, liverish, surface cynicism covering idealism and a sense of noblesse oblige, provides a believable and humanizing contrast to the agile, brash and square-jawed Major (there should be, says another character, "a unit devoted to finding out what Major Maxim is doing and telling him to stop it").

Although in outline the plot might sound farfetched -- Maxim is forced to kill people here and there; he confronts KGB agents in southern Illinois and a right-wing British assassination team in East Germany -- the pace is so fast and the action so cleverly imagined and skillfully packaged that the tension never lets go. According to the publisher, Maxim will soon be seen in a movie and on television, to which his talents seem ideally suited.

Genre fiction is measured by how well it uses conventions. There is, for example, a pervading cynicism common to serious spy novels at least since The Spy Who Came in From the Cold. In The Sisters, neither the CIA nor the KGB is above putting institutional loyalty ahead of national interest. In a wonderful chapter illustrating the Wonderland quality of intelligence gathering, Francis and Carroll, under close observation by a Company interrogator, tell conflicting stories -- one has to be lying -- without challenge by the polygraph machine. Spies galore populate The Crocus List, but they are mostly of a domestic variety: government branch watching branch, department watching department, civil servant keeping an eye on civil servant, a government of employes whose main job is keeping their bosses out of political trouble.

A less palmy tradition of the thriller involves the dragging in, usually from somewhere near the left field fence, of a love interest. Both authors entangle their protagonists in believable affairs, an especially neat trick for Littell whose hero is a dwarfish foreigner on the run. Lyall not only matches up the widowed Maxim with an old flame working now for British security in Washington, but he als does an admirable job of sketching the successful marriage of George Harbinger and his wife Annette. If le Carr,e benefitted the spy novel in no other way, his depiction of George Smiley's relationship to Lady Anne would deserve the thanks of every adult thriller addict.

In short, The Sisters and The Crocus List are first-class entertainments. If the former is not the best of Robert Littell, it is better than the best of almost anyone else. It should find a readership beyond the ranks of thriller fans, whereas Gavin Lyall's arcana, with all its skill and humor, will appeal most to readers who value a good puzzle.