In preparing for the 1980 debate between Ronald Reagan and President Carter, the Reagan campaign staff secretly obtained a copy of the Carter briefing material, according to a book to be published this month on Reagan and his first two years in the White House.

Author Laurence I. Barrett writes in "Gambling with History, Reagan in the White House" that "a member of the Reagan campaign staff somehow acquired an unusual prize: briefing material that the other side was using" before the debate in Cleveland in the final days of the campaign.

Barrett, White House correspondent for Time magazine, says that "apparently a Reagan mole in the Carter camp had filched papers containing the main points" Carter planned to make, except for his reference to his daughter, Amy, and nuclear arms control.

According to Barrett's account, then-Rep. David A. Stockman (R-Mich.), who impersonated Carter for a mock debate with Reagan, "was delighted to find most of his homework done for him as he outlined his own script for the dry run."

The book says James A. Baker III, then a campaign official and now White House chief of staff, "took charge" of preparing Reagan for the debate. But Baker "was grateful not to know the mechanics" of how the Carter briefing material had been obtained, Barrett writes. He does not provide details on how Reagan got the material.

Baker pointed out yesterday through an aide that he did not know how the material had been obtained.