IT IS DIFFICULT to read the latest narrative on Billy Carter's adventures in Libya with a straight face. Here are the Libyans, trying to snuggle up to a president's brother in the hope that kinship means as much to policy in the United States as it evidently does in Libya. Here are the president's brother and his friends, tearing back and forth across the ocean in search of a quick buck. It reads like Laurel and Hardy -- until you get to the end.

This narrative is contained in the interim report (released, stragely, just before the election) by the Department of Justice's Office of Professional Responsibility. That office is trying to figure out who did right and who did wrong in the government's efforts to keep up with Billy Carter's foreign connection. While the report contains little new information, some of its judgments are remarkable.

Attorney General Benjamin Civiletti, his subordinates conclude, was deliberately "dissembling" when he concealed from the press last summer a conversation he had recently had with the president about Billy Carter. His explanations of why he did that and why he initiated the conversation, according to the report, are "inadequate" and "cannot be accepted at face value." Those conclusions are hardly astonishing to anyone who has followed by Billy Carter case closely -- they are almost self-evident -- but it is surprising, and encouraging, that they would be put forward so bluntly by a group of lawyers who work under Mr. Civiletti.

That is only the beginning. The report calls Billy Carter's conduct "reprehensible" -- how else can it be described? -- and says further investigation is required to determine whether he should be indicted for perjury. It calls some of the sworn statements of White House Appointments Secretary Philip Wise "simply incredible" and those of White House adie Thomas Beard "simply not credible." Both men seem to have strangely forgotten and good many conversations about Billy Carter's affairs that other people remember. The investigation is quite properly proceeding to determine what really went on between Billy Carter and these, as well as other, White House staff members.

As for President Carter, the Justice Department's investigators are disturbed that he did not find time during the campaign to be interviewed and dismayed that he has refused to provide them complete access to this personal notes and diary. Here, the investigators are pushing their luck, and their authrity, a little far. The president has made available to them extracts from his notes that this counsel says are complete. Unless there is substantial reason to believe -- as the there was in the Watergate cover-up case -- that those notes contain evidence of a serious crime, assurances of completeness given by the president and his counsel should be adequate. Even a president is entitled to some privacy on notes he has made about matter not under investigation.

The search for legal perfection that these complaints about the the president reveal is also reflected in the report's criticism of Assistant Attorney General Philip Heymann for permitting his staff to publicly about the Billy Carter case. Mr. Heymann did break the Justice Department's secrecy policy, but he did so to fend off the inevitable charges of favoritism and cover-up. This failure of the investigators to give political realities and weight in judging the legal purity of various officials could itself itself mar the Justice Department's final report and undermine the impressive work that the department has done on the whole Billy Carter affair.