Editor's note: This article contains spoilers about Sara Shepard's "Pretty Little Liars" book series.
(CNN) -- In Sara Shepard's universe, being a teenager is truly terrifying.
There's the usual adolescent drama -- fights with friends, heartbreak, parental problems and identity crises -- but there's also an omnipresent stalker and an ever-growing list of dead bodies.
As the author of the young adult book series "Pretty Little Liars," Shepard has taken the typical tropes of young adult fiction and mashed them up with a juicy murder mystery -- one that has now unspooled across 13 novels and has come to life as an addictive, highly social TV series on ABC Family.
At the heart of both the TV show and the books is a group of four teens whose bond is formed by the sometimes deadly secrets they share, particularly those that have cropped up since their Queen Bee, Alison DiLaurentis, mysteriously disappeared one night at the end of seventh grade.
There's Hanna, the shoplifter with a weighty past; Aria, an artsy free spirit; Spencer, a competitive, blue-blooded genius; and Emily, a good-girl swimmer exploring her sexuality. Thanks to the incessant, all-knowing texts from a tormenter named "A," the foursome's high school years in idyllic Rosewood, Pennsylvania, have been deliciously dangerous, both in print and on TV.
Shepard's "Little Liars" first began revealing their secrets in 2006, and so much has happened since then. CNN caught up with the author to talk about her latest "PLL" release, "Crushed," and the TV series adaptation, which begins its fourth season Tuesday night on ABC Family.
CNN: Some of this was inspired by your own teen experience growing up near Philadelphia -- I would love to hear more about what from your own teen years has been incorporated, or fueled the books.
Sara Shepard: None of the scary stuff. They're finding dead bodies; having a barn burn down and having to escape a fire; and having a dead best friend come after them. Obviously that never happened, and hopefully will never happen to teenagers. I was not tormented in any way, I was never even bullied. I had a nice teenage life. It was more of the tame stuff, actually ... and certainly the setting -- a lot of the landmarks I talk about are the same.
The core girls, they're all aspects of stuff that I went through, except for Spencer, she was a little bit more of a good friend of mine. Hanna was me in junior high, wanting to be popular. Emily was me in that middle period between junior high and late high school, being a swimmer and not sure that's what she wants to do and being a good girl. Aria was me at the end of high school, just totally different and over the suburbs and wanting a different life and sick of the lacrosse players.
CNN: The "Liars" are in high school but they attract an adult audience. Were you thinking about that when you came up with this, or did you stumble into a broader audience by accident?
Shepard: It was a little bit by accident. When I was putting this together the really popular book series was "Gossip Girl," and "Gossip Girl" is a lot of teenage people acting like adults. That was sort of the trend that was going on, teenagers living with these adult situations.
I didn't want to write another "Gossip Girl," like a social drama. I wanted it to be deeper, and a mystery, and scary. When I started out, the main problems were things that I saw as a teenager. Hanna is bulimic, and I certainly knew people who went through that. Emily struggles with her sexuality, and again that's something that comes up as a teenager. Aria gets involved with her English teacher. I actually knew of somebody in high school who got involved with her English teacher. It was kind of a rumor, but I have a feeling it was true.
It's dealing with family and friends and love and sexuality and all of that, and I think adults are interested in that, too. I try to keep it pretty juicy and fun. I don't think either the shows or the books are overly "teen."
CNN: You were already several books in by the time ABC Family decided to do a pilot. What was that process like, and going forward, what was your creative input?
Shepard: Right when the series sold at the very beginning, with the first four books, I was told that another network was kind of interested in it, and then I heard nothing about it for many years.
I think it was in October of 2009 that I got a call from Alloy saying that ABC Family was interested. I found out that they had a script, and they were going to make a pilot. It was mostly the same people (from the books).
The amazing thing about the pilot is that it really follows book one -- it's like book one from start to finish. I put so much -- I put a lot into all those books -- but I put so much into that first one, and I was thinking so hard about these characters and this mystery, so to see that on TV was totally surreal.
I really like the show. ... They're their own team, they have a huge team of writers, and I think they just sort of use the books as a guide to the mystery and the characters, but they also come up with their own stuff. I think of it as a parallel universe.
CNN: What are the major differences that you think make it work?
Shepard: I would've loved to keep Toby alive, and I regret that I did not. I really like his character on TV. I thought he was such an interesting character, too interesting to go away. I don't have too many regrets of the series, but that was sort of the only big one. I would've loved for Toby to function in the way that he does in the show.
The (other) thing that I do like about the series is that the girls are a lot closer; they're better friends. In the first four books they're a lot more separate, and it works because they're keeping secrets from one another, but I like seeing them as better friends in the show -- and that possibly could've influenced later books that I've written. Now they are a little bit closer, and relying on each other more.
In book 13, "Crushed," they really want to figure out who A is, so they go into this house, a model home that Spencer's soon-to-be-stepfather has, and they kind of have this big whiteboard where they write down all the suspects of who A might be and narrow it down.
CNN: Sometimes I want to sit down and create a spreadsheet to keep the two plots separate. Is that ever a problem for you?
Shepard: Sometimes I'll watch it (the show) the night that it's on and tweet about it, but often when I'm working on a "Pretty Little Liars" book, I won't.
The mystery is kind of the same, but I don't know where they're going with it. I can't really bring their ideas into it because I have no idea what the end is going to be. I know what my end is going to be, but ...
CNN: Do you have any theories on how this is going to end?
Shepard: I would be so, so excited if Alison were still alive, like in the books. They keep kind of hinting at it, and I know that the writers and Marlene King, the creator, and the people that I work with at (co-publisher) Alloy Entertainment, and the producers on the show, they've all read the books. So I know they're aware that this is a thing -- and it's a huge thing in the books.
But that would be so much fun if she was Red Coat. Otherwise, I have no idea who it could be. Melissa is such an obvious suspect, and so is Jenna -- they're all super creepy and doing awful things, but they're not the one in charge, and it's like, who else could it be?
CNN: What are your thoughts on it being one of the Liars? A lot of people point to Aria -- what do you think about that?
Shepard: I know how I think about it as a writer. I played with that a little bit in book three ("Perfect"), where Spencer worried that she was A. The problem with it in the book is that you have these girls as your narrators, and it would immediately make them an unreliable narrator, and then the reader doesn't trust the character at all -- why wouldn't the character tell us that she has this whole secret life? It doesn't really work for fiction.
I don't think that would be as satisfying to me, honestly, because we love Aria. And I think we want to continue to love Aria, we don't want her to be secretly the person in charge of making her friends' lives miserable. That wouldn't be as fun I think as Alison returning. I hope I'm not spoiling anything, because I really have no idea what their plan is. And I've never asked anybody, because I don't want to know.
CNN: The young adult genre as a whole has changed since you first started. It was a little bit more of a segmented genre where there was young adult and there was adult fiction, and the two were never to meet. But I think it's become so much more fluid.
Shepard: I got an MFA from 2002 to 2004, so right before the series was created. And I thought if I was going to write anything, I was going to write adult fiction. Young adult was never really discussed. That wasn't an avenue to pursue if you were going to be a serious writer, although now I think it really is a lot different. I think YA has changed tremendously in the last eight, nine years.
It's a very different world, and I think there's amazing young adult fiction out right now. It's changed for the better.
CNN: What do you think brought about that change?
Shepard: I don't really know. I know "Twilight" was huge, and that, I think, brought a lot of people to YA. "The Hunger Games" is another one that made reading YA acceptable. It's such a big, hungry market that was kind of unfulfilled as far as I'm concerned -- I'm thinking about what I used to read as a teenager, and there was very little YA back in the '80s and early '90s, so I would just read adult fiction, which was sometimes about teenagers or people in their 20s. If it was good writing then I would read it, but there was nothing that was written well that was aimed at me. It was either fantasy, or "Sweet Valley High." I just wasn't into it.
CNN: Did you have any YA role models? Writers like Judy Blume, do you think she set a path for you in a way? We've come a long way from her kind of fiction though, too.
Shepard: Her fiction was controversial at the time. When I was growing up I read all of her books. I was having a conversation the other day about fan fiction, and I definitely wrote Judy Blume-version fan fiction. Paula Danziger's "This Place Has No Atmosphere," about a girl who is in the future and her family has to move to the moon, I thought was so creative and interesting that I wrote fan fiction based on that. I didn't know that it was called fan fiction at the time because I was in fifth grade and the term didn't exist.
Judy Blume especially sort of broke the boundaries of what is appropriate and what should be written about -- what teenagers are actually doing. "Forever" is the one I always think about -- it was like, "don't read that book!" But it was like,"this is actually happening!
I think her books are timeless, and I hope people are still reading them.
CNN: What's next for you with "Pretty Little Liars"? You said you were contracted for another six after the initial eight.
Shepard: And beyond that I'm doing 15 and 16.
CNN: Have you been approached to possibly turn this into a feature film?
Shepard: A film would be really fun. I wouldn't mind doing another book like "Ali's Pretty Lies," from her point of view, that was a lot of fun. Sort of like a bonus book that doesn't have anything to do with A or the series.
I feel like on TV when the characters go to college it's just no fun anymore, and I don't want that to be the case for the books, but I'm not really sure. There is the fan fiction thing that Amazon just released. They are going to start publishing "PLL" fan fiction, which should be pretty fun.
But me writing more "PLL" books? I hope so. A film would be awesome. I haven't heard anything -- I know readers are always chatting about that, but I haven't heard anything about a film. So we'll have to see.