If anything, he’s speeding up. Two new books bear his name: “True Colors,” a jaded tour of the contemporary art world, and “The Last Party,” a history of “Studio 54, Disco, and the Culture of the Night.” The art book has been well received, but what we really want to read about is Studio 54. And who better to take us behind the velvet rope of that ’70s mecca of disco and substance abuse? A fixture of the subculture he calls “Nightworld,” Haden-Guest is The Last Partier. He’s out more nights than he’s in-a cocktail in one hand, some girl a third his age in the other-dancing the night away in a style Spy called a “strutting and pouting… Mick Jagger-esque boogie reverie.”

Haden-Guest’s notorious excesses might seem unseemly or pointless if he weren’t able to collate them into such perceptive pop chronicles. “The Last Party” is an assiduous mix of memoir and reportage, not as cutthroat as “The Warhol Diaries” but twice as smart. Andy, Liza, Halston, Bianca-all the fading stars who reinvented themselves as “studio celebrities” (famous for being famous) are here. But Haden-Guest is less interested in dishing than in the machinery of nightlife. Like how civilians didn’t go to Studio 54 to gawk at VIPs, for instance; it was the other way around. As he explains over dinner at a stylish downtown boite: “Studio was a place where stars would go and look at ordinary people without being molested.” Eventually the celebrities stopped going out to other people’s clubs and started their own for-profit venues, the depressing outcome of which can be summed up in two words: Planet Hollywood.

Back on Planet Anthony, fun remains the imperative goal. A typically grueling night entails three gallery openings, the 26th birthday party of an aspiring gossip columnist, a dozen random encounters with characters from Haden-Guest’s nightlife, past and present. The final stop is Pop Heaven and Hell, a fledgling clublet where the severely blond manager greets this compact, gregarious Englishman as visiting royalty: “We’re so honored to have you here!” He favors his hostess with some gossip about a club kid recently arrested for murder, while a pair of transvestites hang on his every patrician-accented word.

How does the man do it? Every night! For 20 years. More than 30 years, if you go back to London, where he swung though the ’60s and early ’70s. “I still feel bliss when I’m around people with this kind of energy,” he says. “It’s better than sex.” Must be genetic. Haden-Guest’s mother ran a clubby restaurant of her own in London where boring people were encouraged to leave. His father was a globe-trotting U.N. official – a lord, in fact. Alas, when little Anthony was born, his parents weren’t quite married yet, so the title passed to his half brother, comedian Christopher Guest. Maybe it’s for the best. “Lord Haden-Guest” in one of his Jagger-esque boogie reveries would just make him an even easier mark for the gossip columns.

Not that he’s ever shied away from publicity. The last flurry came three years ago, when Haden-Guest and a girlfriend were both stabbed several times by a man they’d met at a gallery, invited home and offered a couch for the night. “The guy was a well-spoken, believable art-world figure,” Haden-Guest says. Turned out the man was a convicted rapist, and is now in jail. Nightworld has become a darker place than it used to be, Haden-Guest admits. “The violence level has gone up.” When a club magnate was accused two weeks ago of sponsoring kinky “sex parties,” Haden-Guest jumped to his defense, complaining to the New York Post that the city is “becoming more and more like closing time at a museum. I realize that I don’t have nearly enough vices; it makes me want to take up smoking and group sex.” Why not? The night is young.