It’s a hot summer morning, and Jason Zinoman is standing on sacred ground for horror films. The D.C. native is in Georgetown, perched on the very spot where the genre moved from the tawdry backwaters of pop culture into the mainstream of American life.

Of course, the author of “Shock Value,” a new book about how fright films made this transition, is standing at the top of the “The Exorcist” steps.

The steep stairwell at Prospect and 36th streets NW, dropping down sharply to M Street and Canal Road, with the Potomac River and the Key Bridge in the near distance, has become a touchstone for movie fans since the 1973 film in which Father Karras (Jason Miller) hurls himself out of a window and down, down the steps, apparently freeing a possessed child of a demon at the cost of his own life.

“People forget, but the ‘The Exorcist’ was the highest-grossing blockbuster of all time,” Zinoman, 35, is saying in his rapid-fire, enthusiastic delivery. “It was on the cover of magazines. If you wanted to be an intellectual, you had to have a position on ‘The Exorcist.’ You couldn’t just have seen it. You had to have thought about it.”

Zinoman, 35, lives in New York City and freelances theater reviews and articles for the New York Times and Slate. He grew up in the District (his mom, Joy, founded Studio Theatre), and he fell in love with horror films as a kid at Georgetown Day School.

At a friend’s house after school one day, a group of guys watched a tape of “Henry: Portrait of a Serial Killer,” a flick whose widest release was just eight theaters. Zinoman was about 12, and he never quite got over it.

“I went home disturbed,” he laughs.

Mom and Dad were not aware their son was watching such fare, Joy Zinoman says now, with a loud “Yech!” at hearing the title.

“When I was pregnant with him, I directed a play called ‘Marat/Sade,’ which takes place in lunatic asylum,” she says, perhaps by way of explanation. “But I never saw a horror film until his book came out.”

Today, horror is a staple genre, a vast moneymaking machine for Hollywood. Films spawn franchises. Killers become pop-culture references. There are Freddy Krueger, Jason and Chucky, and the ubiquitous “Scream” mask. If politics wasn’t scary enough, Hillary Rodham Clinton’s “It’s 3 a.m.” campaign ad seemed to have clear roots in “Halloween.”

The genre is so entrenched in pop culture that the Web site Box Office Mojo, which tracks ticket sales, separates horror into eight different subcategories (slasher, supernatural, torture, comedy, etc.) and counts more than a dozen films that have grossed more than $100 million. From “Night of the Living Dead” in 1968 forward, the movies have often been made on shoestring budgets. “The Blair Witch Project,” set in Maryland and shot for about $60,000 in 1999, has earned about $248 million, according to the site.

Zinoman’s book, subtitled “How a Few Eccentric Outsiders Gave Us Nightmares, Conquered Hollywood, and Invented Modern Horror,” is about how this came to be.

“The Exorcist” — the biggest of all supernatural movies remains one of the top 10 highest-grossing films of all time — was the brainchild of Georgetown graduate William Peter Blatty. He took his novel to Hollywood, sniffed out a $400,000 offer for the film rights as a bad deal, broke into a producer’s filing cabinet to make a copy of an incriminating document and wound up with as much clout on a film as any writer ever had.

Blatty, now living in Bethesda, is flattered by Zinoman’s attention but loathes how the film has been perceived over the years.

“Exorcist” director “Bill Friedkin and I royally resent the film and the novel being referred to as horror,” he says, laughing. “But we realize nothing we can say or do will ever change that.”

(Blatty says he wrote the book as a devout Catholic, as a means of convincing people that if the devil was real, so was God.)

But the beginnings of “New Horror” had started a few years earlier, when Hollywood’s restrictive Motion Picture Production Code was dropped in 1968. Zinoman writes that filmmakers began leaving behind the “Old Horror” of Frankenstein and Dracula and reaching into the explored territory of “New Horror,” going far beyond anything Alfred Hitchcock ever dreamed. Films could be far more graphic in their violence and much more frightening.

A group of filmmakers — George Romero, John Carpenter, Wes Craven and Tobe Hooper — began dedicated work in the genre, and directors such as Friedkin, Brian De Palma and Roman Polanski worked in the format when the project was right. Steven Spielberg’s film-length directorial break was a 1971 television movie, “Duel,” about a tractor-trailer driver terrorizing a commuter. He became a name in 1975 with “Jaws,” about a shark with a taste for the beach crowd.

“Never in the history of the movies had so much talent been put to work frightening audiences,” Zinoman writes. “The flesh-eating zombie and the remote serial killer emerged as the new dominant movie monsters.”

The landmark films that created the modern genre — “Rosemary’s Baby,” “Exorcist,” “Halloween,” “The Texas Chainsaw Massacre,” “Alien” — had a postmodern sensibility of evil. The directors often refused to give a motive why the monster (real or supernatural) was bent on killing everything in sight, reasoning that the unknown was the most frightening explanation of all.

Zinoman was telling all of this to a crowd of 50 or so at Politics and Prose on a recent evening, treating them to two short, influential films that were almost lost to history.

“Foster’s Release” and “Blood Bath” both involved an unsung hero of the genre, Dan O’Bannon, who went on to script “Alien” and “Total Recall.” O’Bannon played the villain in the 14-minute “Foster’s,” in which a babysitter contends with a madman who calls her on the phone. It turns out that he’s in the house the whole time.

You’ve seen that a million times, he tells the crowd — except this was one of the first times anybody did it on celluloid. A film student named John Carpenter saw this in college, critiqued it . . . and made a fortune on it a few years later with “Halloween.”

“I’m a big horror fan, and he’s someone who takes the genre seriously,” says Drew Harris, a lawyer who speaks with Zinoman after his talk. With him is his girlfriend, Molly Maguire, a lobbyist who makes it clear that watching demonic forces feed on human flesh is not a couples activity at their house.

Zinoman says this is fine, because when horror films are really good, people stop calling them horror.

“ ‘No Country for Old Men’ is about a serial killer, ‘Black Swan’ is about mental terror, and ‘Pan’s Labyrinth’ is about a young girl and the supernatural,” he says. “But nobody calls them horror films. It’s how mainstream the genre has become.”