A HEFTY NEW two-volume paperback, "Unauthorized Transfers of Nonpublic Information During the 1980 Presidential Election," may not make the best-seller list. It's a whodunit, which the authors, the Democratic members of the human resources subcommittee of the House Post Office and Civil Service Committee, humbly admit on the first page doesn't say who did it.

We still don't know who took Jimmy Carter's debate briefing papers from the White House and handed them to the Reagan campaign.

On publication day, the House Republicans sported buttons saying, "Where's the Mole?" They snort that the pilfered pile was campaign material, not government documents.

The tone of the report is restrained, diffident, in fact. The authors simply note that a plague of amnesia afflicted most of the 250 people they interviewed. They complain meekly that some of the answers they got were "less than candid."

Unlike the Justice Department, which conducted an eight-month investigation of the Debategate, and found no "evidence of criminal conduct," the subcommittee chairman, Rep. Don Albosta of Michigan, is not so sure. The diametrically contradictory statements he elicited from the principals suggest,minimally, the possibility of perjury.

Actually, the report does not take us much beyond what we knew a year ago, when in replies to letters from Albosta, White House Chief of Staff James A. Baker III said that CIA Director William J. Casey gave him the briefing book and Casey said he can't remember ever seeing it.

But what these 2,000 pages give us is a panorama of the mindset inside the Reagan campaign. Beginning with manager Casey, whose morbid fear of an "October surprise" from the White House was trumpeted at the time, they were obsessed with the need to know the enemy. In one memo, written by an aide to Edwin Meese, (whose role in Debategate is being investigated by a special prosecutor looking into his credentials to be attorney general) is represented as wanting "more information from the Carter camp." Casey can't remember the instruction.

All the flunkies, consultants, pollsters, hangers-on want to forget the documents they perhaps had triumphantly delivered. They had dozens of them -- Carter's rural strategy, his black strategy, his hostage strategy. Being good Republicans, they saved and filed everything.

In every campaign, a certain amount of spying goes on. To get the drop on the enemy is, afer all, what it 's all about. But in the Reagan camp, it was organized, elaborate, consuming. That might have to do with the conspiratorial character of Casey, an old OSS man, brought up in the tradition of "whatever is necessary.

The real "October surprise," of course, turned out to be Reagan's superior performance in the debate. He was genial, colloquial, at ease. How much the possession of Carter's blueprint contributed to his absence of "malaise" is something that even a special prosecutor might not be able to find out.

Two actors in the huge cast stand out. One is Paul Corbin, an erratic and often surly Democrat, who was once an employe and familiar of Robert Kennedy. The other is Tim Wyngaard, the executive director of the House Republican Policy Committee, the only person who came forward to offer information.

Corbin, through Baker, was brought into the campaign and employed by Casey to talk up Reagan to labor leaders, test the waters in Florida, submit research reports and, of all things, to do the lowliest of all campaign jobs, hand out literature. No reports exist, and a friend says Corbin tried to unload his bumper stickers and posters on him.

Yet he was paid $2,860 by the Reaganites, and the report says, "It appears ... either that Corbin was paid for something else ... or that he neglected to perform the services for which he was paid."

Wyngaard, who was acting contrary to his own interests, is the source of the story that Corbin gave Casey the debate papers. When the story surfaced, Wyngaard went to his boss, Rep. Dick Cheney, and reported that Corbin had told him he did it. Corbin denies it. The report notes, with its customary understatement, that "Corbin's reputation for veracity is uneven."

Reagan still thinks that it is all "much ado about nothing." Naturally, he doesn't want two special prosecutors on the job during the campaign. But Casey is in charge of an agency supposed to be reporting facts and is just climbing out of a mess with the Senate Intelligence Committee, for not coming clean about the mining of the Nicaraguan harbors. The day after the report came out, Reagan called Casey "an inspiration to all of us."

Maybe the president is right and the public doesn't care. Or maybe the new button will say, "Where's the Special Prosecutor?"