IN ANY story about murders and mysteries and "ghosts," there is bound to be a question about "whodunnit".

In the case of Margaret Truman's best-selling "Murder In The White House" and her forthcoming "Murder On Capitol Hill", there now arises the tantalizing question of who-helped-her-do-it? How much has Donald Bain -- a popular ghostwriter whose name seldom appears on book-jackets even though he has 38 successes to his credit -- contributed to Truman's new-found career as an American Agatha Christie?

"Murder In The White House" was on the best seller lists, was serialized in an issue of "Good Housekeeping" which sold out on the newstands and was bought for a made-for-television movie.

Bain denied a telephone interview last week that he had anything "at all to do with" either of Truman's mysteries, beyond having lunch with her and their mutual agent at one point to "chat" about "Murder on Capitol Hill," the thriller which is due out in June.

Truman, in two phone conversations, acknowledged that Bain had been "very helpful" with "dialogue."

She and Bain have the same agent, Scott Meredith, she said, and Meredith arranged for them to have lunch because "I wanted someone to help pull things together."

"We meet at lunch . . . he comes here (to the apartment) . . . we talk on the phone," she said. "I show him dialogue . . . we mail each other things . . . I already have most of the writing done before I show it to him."

Truman has acknowledged in the past that her first fiction was "a collaboration, in a way" between her and Don Fine, Arbor House editor and publisher. Fine, in a New York Times story on Truman last August said "I had something to do with picking who done it," but added his contributions had not included ghosting the book. "Margaret wrote the book," he said.

Truman said that her agent had found Bain for her because Fine didn't have time to help her on this sequel and another for which she has contracted. All three mysteries are to be set in Washington.

Bain, who is well-known in the trade for being little-known outside the trade, has had some big successes in the past. According to an editor for Bantam paperbacks last week, it was really Bain who ghosted "Coffee, Tea Or Me," the adventures of two airline stewardesses which sold millions of copies in the 1960s."

Told that Truman had acknowledged his help, Bain still denied that he has given her any help. "Maybe she's just being generous," he said. "Let her. I can use the credit."

Truman said that she doesn't know about financial arrangements with Bain for his help.

"You'll have to ask Scott Meredith," she said. "I know how much I'm being paid, which isn't much to speak of."

Told that Bain had said she "probably" made $1 million on her first mystery, she laughed: "Don't be silly. I wish I had. I made no money to speak of. There isn't much money in books, you know. But I still have hopes."

According to one source, another book of Truman's which Arbor House is also bringing out in June involves Paulette Cooper, a writer best known for a book called "The Scandals of Scientology".

Truman denies that Cooper wrote the connecting narrative in a book the former president's daughter is compiling of the late Harry S. Truman's correspondence to his family.

"Who wrote it?" Daniel asked rhetorically. "Harry S. Truman wrote it. . . I wrote it. It was just a matter of tying a sentence together here and there."

Cooper "got the letters in order", she said, but did no writing".