"I never see you with clothes on," cried Beverly Sills, smooching Jerzy Kosinski. This, she hastened to explain, was because the novelist and the diva more commonly ran into each other on Martha's Vineyard, wearing bathing suits. "He has great legs," she added.

It was Bantam Books' 40th Anniversary party last night at the Hotel Pierre in New York, and it boasted a room full of best-selling authors. Besides Sills, who's working on a memoir about "the last five or six years of my life, since I've gone over to the other side of the lights," Kosinski schmoozed with Jean Auel ("Clan of the Cave Bear"). "I'm a terrible groupie," confessed Auel, who wanted to know if Kosinski liked the movie version of his novel "Being There." (He said yes.)

Then Kosinski moved on to shake hands with Mario Puzo and Lee Iacocca. "Everybody said everybody loved everybody else's book," reported Puzo's companion, writer Carol Gino ("The Nurse's Story"). "What's [Iacocca] going to say, that he hated my book?" Puzo shrugged. "Am I going to say that I hated his book?"

Iacocca said he felt no pangs when his autobiography, after eight months astride national best-seller lists, was pushed to No. 2 this summer by Chuck Yeager's, which is also a Bantam book. "I get pangs when Chevrolet overtakes Ford or if someone's gaining on Dodge," he said. "The book business is extracurricular."

Louis L'Amour, who arrived in a turquoise bolo tie and a pearl-buttoned shirt, said he was pleased that President Reagan had whiled away his postsurgical recovery with his book "Jubal Sackett." "At a time like that you need something to help take your mind off what's happening," he said. "I was glad I could contribute."