The latest news on the Washington book circuit is that Sen. Edward M. Kennedy (D-Mass.) is negotiating to join forces with former Reagan speechwriter Peggy Noonan to write a book about himself and his brothers.

Because negotiations are reportedly stalled over -- what else? -- the division of the proceeds, few people immediately involved with the project are talking. Not Noonan. Not literary agent Sterling Lord, who is representing the senator in the deal. Not G.P. Putnam's Sons, the publisher with whom the deal is said to be nearly done.

But a Kennedy aide confirms the plans, saying, "Sen. Kennedy has been thinking about doing a book about his brothers and early childhood for a long time. What decided him to move in the direction of action was when he made the decision not to run for the presidency last fall."

The proposed book is described as an account of Kennedy's coming of age with his brothers -- John, Robert, and Joe Jr. Joseph P. Kennedy Jr., a Navy pilot, died over England during a bombing mission in 1944.

Noonan, who once described herself in an interview as "your basic supply-side conservative," is an interesting choice for Kennedy, who has one of the most liberal voting records in the Senate. A Reagan speechwriter from April 1984 until she left the White House last June, Noonan was the author of some of Reagan's most effective, emotional speeches. She is given credit, for instance, for the speech he made Jan. 28, the day of the space shuttle disaster, his address at Pointe du Hoc in Normandy on the 40th anniversary of D-Day, and the speech he delivered in June 1985 at the dedication of the John F. Kennedy Memorial Library.

That was the speech that brought Kennedy to her door. The senator, according to his aide, approached Noonan because "he had become very impressed by the tribute at the library and felt it captures his brother's spirit . . . . They did meet, she was interested."

Noonan said in an interview after that speech that her evocation of Kennedy as "a patriot who summoned patriotism from the heart of a sated country" came easily to her as a longtime admirer of Kennedy and his presidency, and noted that -- her conservatism notwithstanding -- she is "an Irish Catholic with a liberal Democratic background."

When she resigned, citing a desire to "pursue other writing opportunities," many observers suggested that communications director Patrick J. Buchanan had in fact wanted her to succeed Bentley T. Elliott as chief speechwriter, but had lost out to the wishes of chief of staff Donald T. Regan. Regan was reported to feel that both Elliott and Noonan were too ideological in their desire to let Reagan be Reagan.

Does Kennedy see any irony in engaging a co-author out of Reagan's White House? "I really think so," said his aide, "but I don't think he'd say so. Better her than Pat Buchanan." Change of Heart . . .

Reporters noted yesterday that President Reagan's official schedule for the day looked unusually light, listing only a meeting of the National Security Council and a brief speech to White House interns.

The explanation is that the president's handlers had planned some time ago to schedule a news conference last night, and had thus blocked out much of the day's calendar for the normal preparation time. But advisers beat a hasty retreat, White House sources said, after Reagan's speech on South Africa last Tuesday bombed.

Said one source, "We didn't think it would be helpful" to talk any more about South Africa. Close Calls . . .

The Office of Management and Budget (OMB) and the Congressional Budget Office (CBO) are due to report to Congress next month on whether the federal budget for fiscal 1987 brings the deficit within range of the $144 billion figure mandated by Gramm-Rudman-Hollings budget-balancing legislation. The process, confusing at the best of times, is muddled further by the Supreme Court's ruling last month that the comptroller general's legislated role in automatic cuts is unconstitutional.

But all parties will find time this evening to turn their attention to softball. Teams from the two agencies, captained by OMB Director James C. Miller III and CBO Director Rudolph G. Penner, will meet on a diamond across the street from CBO headquarters.

Comptroller General Charles A. Bowsher of the General Accounting Office has been asked to umpire, but both sides have threatened to ignore unpopular calls on constitutional grounds. Please, Mr. Postman . . .

Albert V. Casey, now in his last month as postmaster general of the U.S. Postal Service, said yesterday that he has always questioned the board's decision to hire him in January on what he considered a temporary basis.

Shortly after he was appointed Casey announced his plans to leave the Postal Service, saying that he had been hired only to spend nine months whipping the Postal Service into shape.

But in yesterday's interview, he gave a different account. "I said to the board of governors , 'You should get a permanent postmaster general.' They came to me and they said, 'We are not interviewing you. The job is yours if you want it' . . . . I don't think that's the way it should have been done."

But board chairman John R. McKean has maintained that the board did not intend to hire Casey temporarily. "I don't believe I hired him for nine months. I hired him to be the postmaster general of the United States."

At any rate, Casey is firm in his resolve to step down Aug. 15. The board of governors is expected to name the new chief -- the fourth in three years -- at its meeting next week.