The first ever BookCon, planned as an extension of the mega trade show Book Expo America by the same people who do Comic-Con, took place last weekend. It was headlined by, among other things, a robust diversity debate that bloomed on social media around the hashtag #WeNeedDiverseBooks. But it also functioned as an impressive, invigorating show of force for one of the most important nascent cultural interest groups we have: the Book Girls.

Like any important faction, the Book Girls are not a monolithic robot army marching in step, but a loose affiliation of human beings whose insistent individuality has somehow nevertheless allowed their shared priorities to bubble up and blossom. Who are the Book Girls?

They are readers, and in this particular case, they are girls and women. In fact, one of the sadder things about observing the Book Girls in action is realizing that they – walking from author signing to author signing in happy gaggles, toting friends and sometimes parents – are having their voracious reading habits and their devotion to the importance of talking about your feelings socially reinforced in a way that one fears may be far less common for boys with similar impulses. (Learning that it's okay to talk about your feelings is an important theme for many of the heroines in the books they love.) There are boys and men and older women who love many of the books that the Book Girls do, but it is the Book Girls who scream at authors the way people screamed at the Beatles on Ed Sullivan.

On the young end, they may only be 10 or 11; they remain demographically Book Girls at least through college. And they do, on a broad scale, seen in large groups, seem to emerge as a type that is in a sense unfair to all of them but feels like a weighted average: They dress for comfort; they pull their hair back. They move in groups, they drink iced coffee, they talk about podcasts, I secretly suspect as I eyeball their earbuds that all their music is playlists, and they read all the time. They have The Fault In Our Stars shirts that say "Okay" and "Okay" in word balloons, they are veryglad Harry and Hermione never got together because that would have been terribly reductive, and they consider power and individuality to be topics for books that are at least as important as kissing.

The Book Girls are part of the force that has made The Fault In Our Stars, opening this weekend as a hugely hyped motion picture, such a hit. They grew up on J.K. Rowling, they like trilogies, they embrace dark stories, they are ambivalent about Twilight (they read it, but they're glad Hazel Grace is no Bella), they observe no particular boundaries between high and low culture, and they formed equally overwhelming throngs for Veronica Roth (who wrote the Divergent books) and for Amy Poehler, there to talk about her upcoming book. But in the end, if this was their Comic-Con, John Green is their First Avenger. And he knows it, and he tries to wield that power with some care: he (along with other Book Girl-adored authors) gave a signal boost to #WeNeedDiverseBooks.

And indeed, the Book Girls ponder seriously any issue of fairness and goodness you place before them, and they take it to heart. Roth speaks easily to the Book Girls about the way her thoughts about being a privileged white, straight, well-off person influenced the themes of Divergent; her co-panelist Alex London (who wrote the dystopian YA thriller Proxyand the follow-up Guardian) talked about the sexuality of his characters and why it's important (and not). During his session, Green leapt from the stage to hug a 16-year-old boy (oops — again, defining rough demographic chunks is useful in the aggregate but artificial and flawed as to individuals) with a prosthetic leg who thanked him for answering in TFIOS the question of whether to have sex with the leg on or off. The Book Girls adore Rainbow Rowell's Eleanor & Park, which has an interracial romance at its center, a topic the book treats seriously but does not make into its central conceit.

The Book Girls are only partly real; like most heavily marketed-to demographics, they only sort of exist. Every Book Girl is something else, too – a sportsy girl, a scientist, a nail-art aficionado, a poet, a prodigy, a patient. But the force they are exerting is real. They have created a market for what they love, and they insist upon it. The things marketed to them are not the only things they love –some of the same girls who later showed up at the Roth panel were at the morning panel with John Grisham and Carl Hiaasen, neither of whom is probably being sold with the idea that he's sharing a lot of readers with dystopian YA. They have allies in boyfriends and boy friends, in parents and other adults, in librarians and book critics. The world of their books is much more complicated than just them, and they are more complicated than just their books.

But they, moving and talking and starring and sharing and making fan art and packing paperbacks in their pocketbooks, have helped create a space where girls who fight and feel things are not genre-breaking but genre-defining elements. They can stroll around with The Fault In Our Stars-branded tissue packs, which they acknowledge are funny, stuffed into a Grisham tote bag along with a treasured copy of a novel about death that has a fireball on the cover.

They are voracious and fascinating, curious and powerful, and they have arrived, loudly.

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