– Holly Black and Cassandra Clare walk into the Soho offices of publishing house Scholastic Corp. dressed to kill, in matching black-and-white print outfits. On closer inspection, it becomes clear that while the fabric is identical, the cuts are different — a perfect visual metaphor for the literary collaboration they are about to launch.

The two bestselling young-adult fantasy authors, close friends who live near each other in Amherst, Mass., co-wrote “The Iron Trial,” the first of a series of five novels aimed at readers of middle-school-age and above. They will come to the Fitzgerald Theater in St. Paul on Oct. 1 to discuss the book as this season’s first Talking Volumes guests.

The series follows the adventures of a boy named Callum Hunt, a wiseacre with anger-management issues, who is strong-armed into attending a secret underground school for special-powers prodigies called the Magisterium.

While he might sound similar to a certain bespectacled kid named Harry who goes to a school named Hogwarts, Callum (at least in the first book) is no Potter copycat. Beginning with a startlingly dark prologue and ending with a risky twist, “The Iron Trial” sets its own tone, one marked by a decided murkiness between good and evil.

“There’s a tendency to want to be merciful to your characters, but young readers don’t want that,” Clare said. “They want to experience extremes, so that’s what we tap into. These aren’t your children, they are your protagonists, and they have to suffer.”

Nodding in agreement, Black said she tries to “resist the urge to soft-pedal. You can’t be didactic or elegiac, either. You have to remember how kids process emotion.”

Black’s hair is dyed bright peacock blue, Clare’s brick red, feathering into magenta tips. The easy way they tease and talk over each other, you can imagine that if they had met as tweens, they would have been giggling and scaring each other silly over a Ouija board at a slumber party. As it stands now, the two pals are fond of occasionally mentioning characters from each other’s books in their own work.

Clare, whose real name is Judith Rumelt, was born in Tehran. She spent some time globe-trotting with her parents, including a trek through the Himalayas as an infant, ensconced in her professor father’s backpack. Black grew up near the Jersey Shore. Clare goes on foreign jaunts to recharge her creative batteries, while Black dreams up crafty projects around her Tudor home. Before finding their true callings, Black cut her writing teeth as a production editor for medical publications — including the Journal of Pain — while Clare was a Los Angeles tabloid reporter following the antics of Britney Spears and Paris Hilton.

Each has seen her work successfully transferred to the big screen. “The Spiderwick Chronicles,” based on Black’s middle-grade series of the same name, was the second-highest grossing movie on its opening weekend in 2008. “The Mortal Instruments: City of Bones,” based on the first book in Clare’s “Instruments” series, did not have as strong an opening in 2013, but a follow-up film is in the works, and the new Magisterium series has been optioned by the same producer.

Bookstore bonding

Now in their early 40s, the two met in 2002 at Books of Wonder in Manhattan’s Chelsea Flatiron district, when Clare came to a signing for Black’s first book, “Tithe: A Modern Faerie Tale,” on the recommendation of a friend. A few weeks ago at the same store, more than 100 people showed up, some traveling from other states, for a launch event for “The Iron Trial.”

Bookstore owner Peter Glassman reflected on what makes their partnership so potent.

“Holly has wonderful plotting and pacing, and her settings really give you a feel for places that actually don’t exist,” he said. “Cassie is so passionate, you really feel as a reader that you’re going through what her characters are. What makes them wonderful together is their great friendship. They’re so close that when they merge their styles it’s seamless, capturing the best of both of them.”

Asked whether their subject matter tends to draw out a higher than average number of creepy stalkers, they laughed and shrugged.

“Remember, before we were fantasy writers we were fantasy fans,” Black said. “We know our people.”

Someone did once give Clare “a vial of their own blood, but that was a grown-up, so I’m not blaming a kid for that one,” she said.

They claim that they never had any knockdown, drag-out arguments while working on “The Iron Trial,” which they wrote in the same room, handing the laptop back and forth every few hundred words or so.

“We’ve had more intense fights with each other over books that belonged to just one of us,” Black said. “But I like having people, readers, too, fight with me and tell me what I did to a character was wrong. It means they really care.”

Fantasy’s crossover appeal

In books, at movie theaters and on TV, fantasy and sci-fi dramas seem to be at peak proliferation. With total combined sales of 37 million books, Clare and Black are prime movers in their genre. They link the popularity surge to a breakdown in barriers between these genres and the pop and literary mainstream.

“ ‘Game of Thrones’ wouldn’t have worked 25 years ago; it would have been too confusing,” Clare said. “Now all kinds of people want to follow this alternate world.”

They cited Junot Díaz, Michael Chabon and Jonathan Lethem as prime examples of critically praised literary authors who have written bestsellers for adults that cross over into sci-fi and fantasy territory.

“Audiences have an expectation now that they can move back and forth between reality and fantasy,” Black said. “It frees you to tell any kind of story.”

Social media and online interactive trends also have played a huge part in fomenting the genre’s popularity. Amateur writers of “fanfic” (fan fiction) create new stories around popular authors’ characters, and engage in “shipping,” which is creating new romantic relationships between characters who haven’t been linked that way by their creators.

“We create worlds that teenagers want to live in, and where they can do that is online with like-minded people,” Clare said. “They role-play the characters and build communities around those in the books.”

The duo has found that a shared language unites YA fantasy fans, no matter where they live. Clare recalls being in Brazil on a “Mortal Instruments” media tour and connecting with young Portuguese-speaking fans via their Harry Potter T-shirts and the four houses of Hogwarts.

“I would say ‘Ravenclaw’ and they would say ‘Slytherin’ or ‘Gryffindor.’ And in that moment we knew we shared this imaginary world.”