Short of hiring one of his own characters to rub him out, no mere critic can hope to slow the Robert Ludlum juggernaut. With The Bourne Ultimatum, Ludlum's series of gory, apocalyptic ''megathrillers'' to use his publisher's term now extends to 16 titles. Since the books average as many pages as a good-size phone book, a man determined to read them all at once in the proper setting could log enough frequent-Flier miles to earn a free seat on the first commercial flight to the moon.
No woman on earth would want to read them at all. Unlike many authors in the espionage biz, Ludlum has not been forced by recent world events to retool the production line. Everybody hates terrorists, right? Here the Jekyll/Hyde figure David Webb/Jason Bourne a mild-mannered, 50-year-old professor rebuilt from the psychological wreckage of the CIA's deadliest assassin must reluctantly leave the wife and kids to do battle with Carlos the Jackal, his Soviet-trained counterpart. Renegade Carlos enjoys a profitable freelance career, peddling his vile talent to the highest bidder. This time out, there's a cabal of high-placed, power-mad Americans code-named ''Medusa.'' Oh, and yes, the Mafia also gets in on the act. Ludlum evee throws in an Ivy League law prof whose niche in the legal profession bears more than a passing resemblance to Robert Bork's.
Ludlum's hit-man-with-a-heart-of-gold psychology remains as ludicrous as ever. This is Walter Mitty with an Uzi. As for his style, it's hard to be charitable. He repeats whole phrases like an automated bank teller run amok. Ludlum's word processor must be programmed to type out ''the Director of the Central Intelligence Agency'' with a single keystroke. He never settles for one adverb or adjective where six will do: ''A screaming, pulsating siren erupted, deafening, incessant, echoing throughout the cavernous house like a sonic tornado.''
So why is the reader in seat 18C panting? Because from the ambush that opens the novel to the final bloody extravaganza at ''Novgorod'' a sort of Soviet Epcot Center for spies the pace never lets up. The Bourne Ultimatum is to spy novels as the two-minute offense is to NFL football: You keep thinking Ludlum can't sustain the pace, but he does. C+