A conservative U.S. senator and his former staff aide, in a soon-to-be-released book, have revealed the contents of a crucial, highly classified security directive from President Reagan concerning the "Star Wars" Strategic Defense Initiative.

Surprisingly, the book, "The Arms Control Delusion," was given official advance clearance by the CIA. Either the agency's reviewers overlooked the sensitive quotes, didn't realize how sensitive they were or knowingly approved the book's ad hoc declassification of a presidential document.

The book, scheduled for release Nov. 3, is coauthored by Sen. Malcolm Wallop (R-Wyo.) and Angelo Codevilla. Wallop was a sponsor of antimissile defense programs even before Reagan's 1983 speech gave SDI momentum. Codevilla, an internationally recognized expert on arms control and intelligence matters, is now a senior research fellow at Stanford University's Hoover Institution.

The central thesis of the book is that arms control advocates have sacrificed real U.S. security for the false security of arms control by insisting that the process survive at all costs.

Chapter Six, "The Reagan Administration, Sincerity and Arms Control," contains astonishing excerpts from what the book's authors identify only as "a lengthy, closely held National Security Council document" signed by Reagan.

White House sources, surprised by the document's inclusion in the book, identified it as still-classified National Security Decision Directive 192, signed by the president in August 1985.

The president has said he would restrict certain SDI work, but the directive points out that the negotiating record does not require that the president do this. Here's the stunning, relevant paragraph: "What our recent analyses have led me to believe is that while the ambiguities involved could permit the technical, legal debate to continue, our initial and unilateral assertions about what the ABM Treaty did restrict concerning advanced defensive technologies is not clearly demonstrable in the terms of the treaty as written, nor in the associated negotiating record."

The document seems to be laying the groundwork to justify a looser interpretation of the treaty, to allow work to go ahead on Star Wars. As for the Soviets, the directive states:

"Our assertions about this portion of the treaty were not, at the time, shared by our negotiating partner. Rather, the record indicates that they were resisted by the Soviet Union. These assertions reflected more our hopes for what could result from the treaty . . . than an objective assessment of what was achieved and mutually agreed by the signed treaty document." In other words, the Soviets favored a less-restrictive interpretation from the start, and the narrower U.S. "assertions" were not binding anyhow.

Even so, Reagan's directive continued, SDI research has been "designed . . . to conform to a more restrictive view of our ABM Treaty obligations." He acknowledged that this had been done at the price of speed, overall cost and "technical uncertainty," but insisted that the SDI program, "if consistently {funded} will permit us to achieve the goals set for it."