Author Kitty Kelley thinks she has a couple of good guesses why Frank Sinatra dropped his $2 million suit aimed at preventing her from writing his biography. Kelley says she was ready to subpoena First Lady Nancy Reagan to testify in court that at no time had Kelley represented herself as the singer's authorized biographer.

"I told my lawyers I would subpoena Mrs. Reagan and I know that they told Sinatra's lawyers because they were in constant contact," Kelley said yesterday. "Also, I don't think he wanted my lawyers to take his deposition. Can you imagine me sitting there being able to ask any question I wanted?"

Kelley said, "I think the vision of the first lady of the land being subpoenaed and dragged into court was distasteful to him."

Sheila Tate, press secretary to Mrs. Reagan, said she "never heard of such a thing" as the possibility of a subpoena.

Kelley, the author of the best-selling and unauthorized biographies about Jacqueline Onassis and Elizabeth Taylor, said she does not know if the White House knew of her intentions to subpoena Mrs. Reagan. But she said that nowhere in written requests for an interview with Mrs. Reagan did she say that she was Sinatra's biographer.

"I do know that Frank Sinatra is very important to this White House, that the Reagans have made him director of entertainment for state dinners. And I am trying very hard to interview Nancy Reagan about him as well as other people who know and love him," said Kelley.

She said she wrote two letters (Jan. 24 and Feb. 10) to the White House asking for an interview. Tate answered the first letter by saying Mrs. Reagan was too busy at the time, Kelley said.

"In my second letter I said I would be more than happy to wait until after the campaign, but I never heard a word," Kelley said.

Tate said, "We knew what she was doing, that she doesn't write official biographies. After she released her second letter to the press, I didn't feel the need to respond."