President Reagan yesterday denounced a new book describing secret operations conducted by the late CIA director William J. Casey "as an awful lot of fiction about a man who was unable to communicate at all." But Reagan confirmed that he had signed a secret order authorizing counterterrorism actions in Lebanon.

When asked whether Casey, the director of the Central Intelligence Agency until last January and who died in May, had ever carried out covert operations without his knowledge, the president said: "Not that I know of."

Reagan's comments on the book by Bob Woodward, an assistant managing editor of The Washington Post, were made during a picture-taking session in the Roosevelt Room. The book asserts that Casey circumvented normal CIA channels and arranged for the Saudi Arabian intelligence service to undertake three covert operations, one of them a failed March 8, 1985, attempt to assassinate Sheik Mohammed Hussein Fadlallah, the leader of the Shiite Moslem faction known as Hezbollah, or "Party of God."

Instead of killing Fadlallah, the car bomb used in the operation exploded in a Beirut suburb, killing 80 persons. Reagan was asked if he had signed "the directive that led to a massacre in Beirut."

"No, and I have a copy of the measure that I signed," Reagan replied, adding that it was "nothing but . . . a plan requested of us by the government of Beirut -- of Lebanon, I should say -- to help them in counterterrorism. Never would I sign anything that would authorize an assassination. I never have, and I never will, and I didn't."

The book, "VEIL: The Secret Wars of the CIA, 1981-87," does not say that Reagan signed a directive authorizing the attempt on Fadlallah's life or say that the Lebanese requested a counterterrorist operation. It describes the presidential finding obtained by Casey as one "that would direct the CIA to train and support small units of foreign nationals in the Middle East which would conduct preemptive strikes against terrorists," a plan that was to begin in Lebanon and expand later into other Mideast countries.

Later, Woodward writes, Casey became frustrated with the slow implementation of this plan and turned to Saudi Arabia for help in the secret attempt to assassinate Fadlallah.

Reagan's complete answer to a question about his reaction to "Casey's off-the-books" operations was: "I think that there's an awful lot of fiction about a man who was unable to communicate at all and is now being quoted as if he were doing nothing but talk his head off."

Woodward writes that he talked with Casey on 48 different occasions over a three-year period while working on his book, the last time in a room in Georgetown University Hospital here "several days" after Casey resigned on Jan. 29. Woodward wrote that in this brief meeting during which Casey spoke 19 words and had obvious difficulty speaking, Casey "nodded yes" when asked if he had known about the diversion of proceeds from the U.S. arms sales to Iran to aid the Nicaraguan contras.

Since the first accounts of the book appeared last week, administration officials have suggested that Casey was "unable to communicate at all" after he was operated on for a brain tumor, as Reagan put it yesterday. The former CIA director died on May 6 after contracting pneumonia.

But officials gave a different picture of Casey's condition during the period in which Woodward said he had his last conversation with the ex-director.

On Jan. 27, chief CIA spokesman George Lauder said that Casey was "requesting and receiving and absorbing briefing materials." The New York Times on Jan. 28 quoted an unnamed administration official as saying that Casey's speech was returning to normal and added, "The impression of Bill Casey lying comatose in the hospital just isn't true."

Two days later, Casey submitted his resignation to then-White House chief of staff Donald T. Regan and Attorney General Edwin Meese III. Sophia Casey and Robert M. Gates, CIA deputy director, also were present in the hospital room and officials said subsequently that Mrs. Casey was the person who signed the resignation letter.

In announcing Casey's resignation on Feb. 2, White House spokesman Marlin Fitzwater said Casey "continues to improve steadily" and "is alert and has visited other patients." During briefings this week, Fitzwater has said he does not know details of the resignation meeting but said he was told at the time by Donald Regan that Casey "offered" the resignation.

On March 20, a source knowledgeable about the events leading to Casey's resignation told The Washington Post that Gates spent most of January trying to fend off demands for a resignation until the CIA director decided to quit or to return to his job. This source said that Gates visited Casey in the hospital on Jan. 28 and found that he was "lucid . . . clear" but "couldn't talk that well."

The source said that Casey told Gates that it was time for him to get out of the way and make room for someone else. Gates then arranged for Regan and Meese to visit the hospital the next day and accept Casey's resignation, the source said.

Fitzwater said that Reagan had telephoned Mrs. Casey to "reassure" her on Monday, the day the book was published.

In response to a question, Nan- cy Reagan said yesterday through

her press secretary, Elaine Crispen: "I find it distasteful to have things written about someone who's dead and who has no chance to reply . . . . "