Avon Books this week bought the paperback rights to David Stockman's "The Triumph of Politics: Why the Reagan Revolution Failed." Neither Harper & Row (the hardback publisher) nor Avon would confirm the sale figure, said in New York publishing circles to be $180,000. When the book was published at the end of April, it shot to the top of the best-seller lists. This Sunday it will be third on The New York Times' list, and publishing sources expect the decline to continue. It remains No. 1 on the Publishers Weekly list.

The paperback sale figure is considered respectable but disappointing. Stockman is out of the country and was unavailable for comment. Harper & Row bought the book for $2.3 million. By comparison, Berkley Publishing last week bought the paperback rights to comedian Bill Cosby's "Fatherhood" for $1.6 million. Cosby's book will be at the top of this Sunday's New York Times list. In addition to Avon, only St. Martin's is known to have entered Tuesday's bidding on Stockman's book. Bantam Books, one of the major paperback publishers, never considered bidding on it. One source said, "If you ask me, 'Does it have legs?' my answer is no."

Royal Watch

Queen Elizabeth II is back in the United States. She arrived at Lexington's Bluegrass Airport yesterday for another of her private visits to the bluegrass country horse farms. The queen, a respected horsewoman, takes an active role in the management of her British stud farms and racing stable. She has several broodmares at Kentucky farms being serviced by top American sires.

As on her similar visit to Kentucky in 1984, the queen will stay at William and Sarah Farish's Lane's End Farm outside Versailles, Ky., which has since become the home of 1985 Kentucky Derby winner Spend a Buck. This visit, like the 1984 trip, is private, and the only opportunity the public will have to see the queen is today at the airport arrival and greeting by Kentucky Gov. Martha Layne Collins, at church services Sunday at St. John's Episcopal in Versailles, and when she leaves from the airport Monday.

End Notes

Sportswear designer Perry Ellis, who has been ill for several months, is in serious but stable condition at a New York hospital suffering from encephalitis, an inflammation of the brain, a hospital spokesman said . . .

Congressmen get a bit carried away with sports. There they were tossing basketballs around on the House steps of the Capitol, Texas Reps. Jack Fields and Mickey Leland and Massachusetts Reps. Silvio Conte and Ed Markey, who even had a couple of live lobsters with him. It had nothing to do with legislation, just some back-home pride about a basketball championship. Starting Monday the Houston Rockets and the Boston Celtics begin their championship series, and the home-team congressmen made bets of lobster dinners against steak dinners, with the losers giving the winning lawmakers a wheelbarrow ride around the Capitol . . .

*Academy Award-winning actor Burt Lancaster has joined an AIDS awareness campaign in which he will pose for a red, black and white poster, holding a rose and advising readers, "Think before you act." It marks the first project the 72-year-old actor has become involved in for the organization Aid for AIDS. The poster is aimed at increasing AIDS awareness among high-risk groups . . .

If you think you've seen Sylvester Stallone movies, you haven't seen anything yet. The 39-year-old superstar of "Rocky" and "Rambo" fame today announced a multimillion-dollar agreement with United Artists to make 10 films in six years. He will star in at least five of them, and his production company, White Eagle Enterprises, will develop, finance and produce them all. United Artists will handle marketing, distribution and sales. Stallone, the No. 1 box office star in the world, can boast that his movies have grossed more than $1 billion . . .