Every social movement needs someone to tell its tale, and like an urban Virgil, Greg Sherwin is the self-appointed guide to San Francisco’s increasingly obsessed coffee culture, chronicled in “The Brave New World of Coffee." Since 2003, the UC-trained bioengineer and Chicago transplant has visited more than 500 San Francisco cafés—many of them fetid and fluorescent-lit—and sipped their espressos, assessed their flaws and virtues, and ranked them online. Sifting through links on Sherwin’s blog, theshot.coffeeratings.com, you get the impression of a droll, diligent man whose persistence is matched only by his capacity for self-abuse.

On the bleakest of his journeys, Sherwin ferried himself to a Market Street outpost of Lee’s Deli, where he forced down a beverage as black and foul as Orc’s blood. The “nastiest espresso” in the city, he declared. Prospects grew no cheerier downtown at Happy Donuts (“sad espresso”) or at Hungry Joe’s on Church Street, which traffics in a “flat and metallic” beverage bursting with the flavors Sherwin might have encountered by sticking out his tongue and “licking the machine.” It’s all enough to drive a coffee lover to despair, although Lee’s no longer serves espresso, and Hungry Joe’s has since gone out of business.

But just as the reader begins to lose all hope, Sherwin’s reviews grow brighter and more upbeat, with hints of clove and a hazelnut finish. Full body. Robust crema. Chocolaty aroma. You get the picture.

“When I started,” Sherwin says, “there was plenty of terrible coffee out there, and there still is. But there are also more people doing it right.”

Blue Bottle Café, which Sherwin calls no less than a “coffee cathedral,” always receives his highest ranking. “When you come across them, it’s a revelation. You think, ‘Ah, so this is what it’s supposed to be.’ ” Drop by for an espresso, then read Sherwin’s descrip­tion of its “flavor of roasted tobacco with an edge of sweeter honey.” You can almost understand what he’s talking about.