The case of Carter's briefing book

A radio commentator invoked memories of Watergate this week when he caught up with the report of that alleged 1980 ''filching'' of a White House briefing book by a Reagan campaign ''mole'' in the Carter administration. It was the kind of easy overreaction not surprising when heard earlier from former Carter aides in full righteous - and partisan - indignation. But people in the news media cannot have similar allowances made for pumped-up perspectives.

Congress can help to keep the public's perspective accurate by proceeding promptly and thoroughly with the congressional inquiry now under way. This week the effort was

broadened to include CIA Director Casey along with the three other Reagan associates - James Baker, David Stockman, and David Gergen - previously asked to explain the episode.

So far it appears that no one has denied that the Reagan campaign had the briefing book, which was allegedly used to help Mr. Reagan prepare for his debate against President Carter. A House subcommittee inquiry is trying to find out how the papers were obtained and whether legal wrongdoing was involved.

It would do no one any good to let suspicions linger.