Probably every journalist in the country has at least one Doonesbury cartoon taped to the wall. A case can be made that G. B. Trudeau, unencumbered by a need to hew strictly to facts, can find the essence of a news event with a precision the columnists can't match. Perhaps the 1974 recession really was caused by an oil executive who simply got behind in his mail (''pure carelessness''), and anyone who's seen ''Love Boat'' can believe that NBC was once programmed by Freddy Silverman's young nephew, whose latest idea involves the discovery by a former president's wife of Elvis's clone aboard a UFO near the Bermuda Triangle.
It has reached the point where each new Doonesbury collection is not so much reviewed as announced. So here is his latest, a best-seller, with strips from spring 1978 through spring 1980, including the famous coupon readers could clip and send to Tip O'Neill, asking about the involvement of the speaker of the House with Korean bribe meister Tongsun Park. ''After the tenth bag of postcards was carted over to the speaker's office, the post office was alerted to stop delivery,'' says Trudeau. ''Now that's lobbying.''