A book author's biggest job used to be writing. Now that may only mark the beginning of the job. Promotion, quarrels and intra-publishing fights often take up an author's time after publication. Consider these two Washington free-lance writers.

Kitty Kelley, 36, wrote Jackie Oh! , an unauthorized, gossipy biography of Jacqueline Onassis that some consider an insult to the former first lady. Kelley's book coincided with the publication of a more flattering book, Jacqueline Bouvier Kennedy Onassis by Stephen Birmingham.

Dan Moldea, 28, lived with friends for four years, taking occasional jobs with NBC and the Detroit Free Press as he researched the life of Jimmy Hoffa. His scathing biography, The Hoffa wars , was released a week after Simon & Schuster published The Teamsters , a more sympathetic look at Hoffa by Esquire's legal affairs columnist Steven Brill.

Both Kelley and Moldea angered a large, and perhaps vengeful, audience. Both have defended their works against hostile reviewers, interviewrs and, sometimes, fellow writers. But both are enjoying respectable sales, especially Kelley, who says 150,000 copies of her book are in print; Moldea says a third printing brings his total to a respectable 54,000 copies.

Footnote: The two authors, who are not acquainted, have another similarity: both worked at different times, in different places, for the presidential campaign of Eugene McCarthy.

Kitty Kelley probably smiles all the way to the bank when she thinks about the fans of Camelot her book has outraged."I guess," she sighs, "it's better to have a big enemies than small ones."

Kelley's enemies are watching her get rich from sales of Jackie Oh! The hardback book, published by Lyle Stuart, is on best-seller lists, and paperback rights reportedly just sold for about $500,000.

On talk shows around the country she is greeted by hostile questioners who ask if her report that the former first lady once underwent electroshock treatement and that Jack Kennedy was more of a Romeo that has been previously revealed . . . if all that smarmy insider's stuff isn't jst too tacky to print.

"This is a gossipy book, yes," Kelley answers. "But it just has one redeeming feature: it all happens to be true. It's documented. Not one person has come forward to deny their quote."

But one source for some salacious Kennedy stories in Kelley's book, former Florida senator George Smathers, grows nearly apoplectic at the mention of her name.

"Kelley is just a pain in the ass in addition to being a substantial provacteur," he says.

"Jackie has read the book," Kelley says, "and she despises me. But she really hates Smathers. Ooooh, what an interview that was! I wrote him about 20 letters, made 30 phone calls. I talked to him in his Washington law office, with the door open, feet on the floorn and no drinks."

"As far as I'm concerned, her feet will always be on the floor, and the door will always be open!" responds Smathers, who says he finally agreed to meet with Kelley only because she said she had spent some time in a spa with his first wife and because she promised her book was going to be "a eulogy of Jackie."

Nevertheless, Smathers refuses to say that anything attributed to him in Jackie Oh! is inaccurate.

To those who disparage gossip, Kelley says: "If it hadn't been for gossip, we wouldn't have three books of the Bible because Mark, Luke and John never knew Jesus. They wrote 100 years after the guy died. At least I got to people who are still kicking."

During the years he researched The Hoffa Wars , Dan moldea was beaten up once and twice told he would be killed. His original publisher, New Republic Books, balked at releasing the book prompthly when publishing giant Simon & Schuster - which distributes New Republic's books - announced it planned to issue its own book on the Teamsters by Steven Brill.

So Moldea signed a new contract with British-based Paddington Press and now says he's at war with "publishers, authors and members of the press who still lionize Hoffa." Moldea considers Hoffa "the moral equivalent of a child molester" who was the link between organized crime and the CIA during the years the federal government plotted to kill Fidel Castro. He further suggests Hoffa's hatred for the Kennedys makes him a prime suspect in the assassination of John Kennedy.

Moldea's detractors include labor reporters who argue that Hoffa wasn't as evil as Moldea claims and that Hoffa's tormentor, Robert F. Kennedy, wasn't as righteous as Moldea portrays him. Of course, Teamsters who rever the memory of Hoffa don't think much of Moldea either - the Washington headquarters of the union labeled Moldea's book "fiction."

On the promotion circuit Moldea even has to answer anti-union zealots who use The Hoffa Wars as ammunition against organized labor.

"I'm a strong trade unionist," says Moldea. "I just happen to be in favor of honest, clean unions that are democratic and run by the rank and file."

An NBC news producer, familiar with moldea's years living hand-to-mouth while researching the Teamster, calls him a "guerrilla writer." Moldea likes the term, enjoys thinking of himself as an activist, and cooperates with investigators who would like access to his nonconfidential files, tapes and sources. But what does guerrilla writer do when the excitement fades?

"I think," says Moldea. "some frivolous fiction is in order now, something a littlshorter and a little safer."