FEAR OF FLYING propelled KK Sharp into reading spy thrillers - anything for diversion at 30,000 feet. And since her husband, Philip Sharp, is a Democratic congressman from Indiana she went through a lot of them, traveling with him to his district. So when she decided to write a novel, a spy story seemed "the easiest, most structured way to begin," she says. For a couple of years, she wrote only when she felt like it - amid political campaigns, household moves and having a baby (he's now two and a half). It wasn't until last October, when her spy thriller was sold to Richard Marek (who has his own imprint at Putnam) and to Macmillan and Pan Books in England - and she found herself with a September 1 deadline - that it began to seem like work. Now - except for details like what to call it and how to sign it (she's officially Marilyn Augburn Sharp, but has been known as KK all her life) - she's finished. About the plot, she will only reveal that "It's a spy thriller set partly in Washington and around the world." The rest is strictly top secret until publication day next February.