When White House curator Clement Conger got a letter last year from Margaret Truman Daniel, asking him to send her a detailed architectural floor plan for the private second and third floors of the Executive Mansion, Conger obligingly sent the drawings along without asking why she wanted them.

She was trying to figure out a place to murder a secretary of state.

President Harry S. Truman's daughter has written a whodunit, titled "Murder In the White House," which Arbor House is publishing in June. The book has already been sold as a television movie to be produced by Dick Clark.

Daniel devoured murder mysteries before she decided to write one herself.

Her husband, retired New York Timesman Clifton Daniel, is also an avid mystery fan. He is so good at analyzing the craftsmanship involved that he once spotted a mistake in the clues that solved one of Agatha Christies' best-sellers.

He wrote to tell her of the error and Christie wrote back that he was absolutely right. Neither Christie's husband, who read the original manuscript, nor her publisher had caught the mistake.

Corrections were made in the next printing.

Which presidential candidate has recently been in touch with former CIA agent John Stockwell, offering to set up lecture and television appearances and "make opportunities" for him to continue to "be critical of the CIA and President Carter?"

Stockwell won't say. Not till he has decided whether or not to take him up on the offer.

The Justice Department filed suit yesterday against Stockwell for the profits from his 1978 book, "In Search of Enemies," which blew the whistle on CIA activities in Angola.

Earlier yesterday, Stockwell said that if the government sued him, he would probably accept the candidate's offer and spend the next year on the lecture circuit, "raising hell."

Stocking is currently busy in two television projects. One, sponsored by the BBC and WGBH, the public television channel in Boston, is a film study of Cuban activities in Africa.

With a television producer, Stockwell has made a couple of trips to Cuba and is waiting now for visas from the African countries involved.

Kentucky Gov. John Y. Brown's wife, Phyllis George, has formed a Kentucky Film Foundation and is expecting to raise about $25,000 for her new pet project tonight at a premiere of "Coal Miner's Daughter" in Louisville.

Brown is bringing in the star, Sissy Spacek, and the subject of the movie, Loretta Lynn and, it seems, half of Hollywood. The foundation hopes to convince movie-makers to shoot more and more of their locations in the varied backgrounds of race-horse country and Appalachia. "Coal Miner's Daughter" was shot in Neon, Ky.

ABC's "20/20" will film the premere.

Brown is not going to let bluegrass grow under her feet. On St. Patrick's Day, there will be another influx of VIPs, with Ed McMahon leading a parade coming to celebrate the Browns' first wedding anniversary.

That night, the Browns will give a dinner in the executive mansion in Frankfort.

In between, Mrs. Brown is running around to places like Possum Trot, Ky., lining up craftspeople for a shop she is opening on Madison Avenue in New York to promote the state's handmade wares.