The most plausible happening in this, the sixth installment of The Ludlum Best-seller, is the murder (yes, it was murder!) of J. Edgar Hoover by ""Inver Brass,"" code name for a sextet of Olympian Washingtonians. It's after that that things get stretchy--even for fans of the fanciful. Seems that the folks from IB only found the first half of what, with good intentions, they were after: the rumor-famous Hoover dirt-on-everybody files. Somebody beat them to M-Z and is using the file's kinky secrets to blackmail and control members of Congress, the military, and the fourth estate. Inver Brass' solution? Inveigle Peter Chancellor, Ludlumish bookman, into researching' and writing the Hoover truth as fiction, thereby teasing the thief-blackmailer out of hiding, which, of course, turns Chancellor into an endangered species. Huh? You may well ask, for even though the usual tough-hyper concerto for bullets, cars, and double-cross is played loud and fast, the strains of too many encores are showing. Will that mean less interest than usual come March? With that track record? Don't be funny.