Though they spend years pawing through the lives of others, it is often said that biographers are actually trying to learn something about themselves. Perhaps the same goes for historians, because in Gary Krist’s well-researched account of New Orleans’ turn-of-the-century underworld, there is no mistaking his affection for the ribald and rowdy.

“It was a strange and disturbing place to many,” Krist writes, setting the stage. “Brothels specialized in all manner of interracial mixing and arcane sexual practices, while narcotics, alcohol and loud, degenerate music filled the saloons and dance halls, promoting deviant behavior of all kinds.”

You can practically hear him chortling with glee.

As profiled in “Empire of Sin: a Story of Sex, Jazz, Murder, and the Battle for Modern New Orleans,” society’s black sheep dominated Crescent City commerce at the turn of the 20th century. Yet these prostitutes, pimps, saloon owners and “sporting men” occupy a certain moral high ground in Krist’s earthy tale.

At least they’re honest about their off-color tastes — so much that prostitutes marched in for work shifts just like any other employee, arriving for what city native Louis Armstrong later described as a virtual assembly line of sex. These denizens and their customers, happy to mix socially with any and all, were also among the nation’s more racially progressive citizens. Meanwhile, a battalion of prim, self-appointed reformers unabashedly favored sterilization for the lot.

Krist’s tale, which opens with the decision of famed madam Josie Lobrano to “go respectable,” traces the 30-year battle between these warring camps. Lobrano’s aspirations never fully take hold, and her trajectory from brawling street hooker to (semi)-respected businesswoman parallels the city’s battle with its own self-image.

Woven into this colorful history are the stories of a young Louis Armstrong, ascendant ragtime legend Jelly Roll Morton and infamous madam Lulu White (said to be the inspiration for Mae West’s raunchy screen persona).

One Tom Anderson — oilman, state legislator and consort to several prostitutes — lords over it all as an “Andrew Carnegie of the carnal,” the unofficial mayor of New Orleans’ red-light district.

Krist, author of “The White Cascade,” a widely acclaimed disaster narrative chronicling the deadliest avalanche in Washington history, is an enthusiastic raconteur of the forgotten. He tears through diaries and old newspapers, bringing fleshy life to what might otherwise dissolve into sepia.

There is even a possible serial killer who prowls from home to home, butchering families while they sleep. The identity of “the axman” remains a mystery even today, though Krist offers some plausible theories. But any reader searching for tales of an American Jack the Ripper will likely be disappointed. This storyline doesn’t develop until the final chapters and feels like an add-on.

Even recounting the axman’s bloodcurdling crimes, Krist offers a knowing wink, deftly tying the murders to New Orleans best-known export: jazz.

“I am very fond of jazz,” the axman says in a letter to the newspaper, written to tell the terrified populace how they might save themselves: “Those persons who do not jazz it on Tuesday night (if there be any) will get the ax.”

This, apparently, was all the excuse that irrepressible New Orleanians needed. On the evening in question, house parties sprang up, filled with jazz all night long.