Perhaps one of the unlikeliest best sellers ever, “Fast Food Nation” – a scathing ex posé of the (literally) filthy underbelly of the American fast-food industry – turned its author, Eric Schlosser, into a hero among cranky college kids (who see him as their version of Ralph Nader) as well as average, non-uppity Americans.

Rather than being a scolding tome about the perils of fast food, Schlosser’s book was filled with vivid descriptions of the often surreal scenes he found during his investigation.

“Upton Sinclair’s ‘The Jungle’ pales in visceral impact,” wrote Nicols Fox in the Washington Post, “as Schlosser details the blood- and brain- spattered horror and interviews the meat-industry’s maimed and abused throwaway workers (meatpacking is now the most dangerous job in the United States), mostly immigrants and minorities, who are now worlds away, in terms of pay, safety and job security, from the relatively high-paid, skilled workers of a generation ago.”

Two years after its publication, “Fast Food Nation” remains a bestseller in paperback; ask anyone who’s read it, and they’ll likely tell you that they will never eat fast food again.

Now Schlosser is back with his second book, “Reefer Madness: Sex, Drugs and Cheap Labor in the American Black Market” (Houghton Mifflin, $23).

It’s a collection of essays about three of Schlosser’s latest obsessions: marijuana laws that he believes to be overly punitive; the rise of the modern pornography industry; and the state of poor migrant workers on California strawberry plantations.

While “Reefer Madness” may not find as enormous an audience as “Fast Food Nation” did, it deserves one: Schlosser, 43, is a meticulous reporter and an elegant storyteller with a keen eye for hypocrisy.

The book is divided, by subject, into thirds. In the section dealing with marijuana, Schlosser introduces us to Mark Young, a pot smoker from Indiana who acted as the middleman in one deal and wound up with a life sentence.

Around the same time, the son of Indiana congressman Dan Burton (who once introduced legislation requiring the death penalty for drug dealers) got busted for growing marijuana in his apartment.

His sentence was community service, probation and house arrest.

While you might expect the author of such tracts to be a rabid activist who spends all of his free time railing against social injustices on college campuses and at public rallies, Schlosser is actually a soft-spoken intellectual (he’s a product of the Dalton School and Princeton) who lives on the Upper East Side with his painter wife and two kids.

And he seems quite at home in the genteel lobby of the Algonquin Hotel, where he sat and talked about “Reefer Madness,” his unlikely writing career and his next target.

How would you describe your politics?

I like to believe they are complex and unpredictable. I definitely have a compassion for the poor and an anger at injustice.

But a lot of conservative Republicans are braver and more honest than liberal Democrats on the drug issue. William F. Buckley and the National Review have been very blunt about how crazy the war on marijuana is.

Do you get any heat for your views?

Since “Fast Food Nation,” I’ve received some hostile hate mail. Mostly it’s people saying I’m a communist and I should go back to Romania or whatever. Every now and then, one of them will show up at a reading. But it’s not that bad.

How did you get interested in marijuana laws?

First, let me make it clear that I’m not a “cannibis is sacred,” kind of person. I just think the angry debate over marijuana should be brought down a level. The war on drugs imprisoned 724,000 people in 2001 alone. Some people are facing 20 years in prison for selling a glass water pipe with a pot leaf on it. It’s unbelievable.

What do you think the solution is?

Decriminalization. Everyone would benefit, even people who have never smoked pot. The billions of dollars currently being wasted in our criminal justice system by locking up pot smokers could be used to prosecute much more serious crimes.

What’s your next book?

It’s about prisons. When I visited Mark Young at the federal penitentiary in Leavenworth, Kan., it was the first time I’d ever seen one, and it affected me. These are scary places, and they’re especially bad because the system is so horribly overcrowded. Two million Americans are incarcerated in a system built to accommodate many less.

Why should we care?

Prisons create a class of people who get out and affect us all. Most prisoners go in as dysfunctional people, but they come out really dysfunctional.

How did you get into journalism?

I was an unsuccessful novelist. I still have an unpublished novel in my drawer – it’s a sort of “Bonfire of the Vanities” about the greed of 1980s New York, with a complex mix of characters including a naive newspaper reporter who gets swept up in that world. It’s supposed to be funny.

You were also a screenwriter, right?

I worked as a script reader for Tribeca Films, Robert De Niro’s company. I read unsolicited scripts and wrote synopses. Then the company gave me a chance to work on my own script. I worked with another writer on a mobster picture set in contemporary Chinatown. The idea was to do for the Asian-American gangs what “Goodfellas” did for the Italian-American mob.

What happened?

It didn’t get made, but I got to do a lot of research in Chinatown. I discovered a world that I didn’t know about. I remember walking into this old tenement building and seeing rows of bunkbeds. The workers were literally renting their beds for eight hours. When they got out of bed, someone else got in it.

That sounds like the sort of research you’re doing now.

Yes. Some of my writer friends [including college pals Katrina vanden Heuvel, editor of The Nation, and John Seabrook, a staff writer at the New Yorker] suggested that I should try journalism. I broke in with an unsolicited query to The Atlantic [Monthly].

Your father is the former president of NBC, Herbert Schlosser, who helped create “Laugh-In,” “Star Trek” and “Saturday Night Live.” What does he think of your work?

He comes from a milieu that was very concerned with social issues. He used to put documentaries on the air in prime time – and I don’t mean documentaries about Michael Jackson. Real ones. I think he would have loved it if I had gone into public service. But he also likes that I’m writing about these issues.