Editor Donna Bray remembers exactly how she heard about I’d Tell You I Love You, but Then I’d Have to Kill You, first in a series of books about a group of girls attending the Gallagher Academy for Exceptional Young Women, an elite school for training spies.

It was 2005 and Bray, then working as an executive editor at Hyperion Books (now co-publisher of the Balzer + Bray imprint at HarperCollins), was at a conference when her assistant called to tell her about the first pages of an exciting project that just arrived. To save on fax charges at the hotel where she was staying, Bray had her assistant resize the 60-page document into 25 pages of single-spaced 10-point type. “I still loved it,” she said, recalling reading the tiny type in a hotel room at 11 p.m. “I was completely taken by the voice. I thought the concept was brilliant and smart.” She made an offer right away, signing the author to a three-book deal.

Since Ally Carter made her YA debut with Hyperion in 2006, she has become the queen of the “clean” teen series, writing books without much in the way of sexual content or strong language. In addition to the five already published Gallagher Girl book (Bray edited the first three), she has written three installments of the Heist Society, a series about a girl from a family of thieves who gets embroiled in her own high-risk jobs. Between the Gallagher Girls and Heist Society books, Carter has sold more than two million copies, and both of her series have been optioned for film.

On September 17, the sixth and final Gallagher Girls installment, United We Spy, arrives on bookshelves. Carter will bid goodbye to the Gallagher Girls with a multicity tour this month, which wraps up, appropriately, at Washington D.C.’s International Spy Museum on September 29.

Cracking the Code

The Gallagher Girls may speak more than a dozen languages, take classes in encryption and covert operations, and take on a terrorist organization, but at the core, they are classic YA heroines.

“They are extraordinary girls who go to an extraordinary school, but they are still grappling with things that teen girls have to deal with,” said Carter’s longtime agent, Kristin Nelson at Nelson Literary Agency.

Though the books deal with crushes and decoding boy language, she said, friendship and strong female role models remain central. “It’s much more Sisterhood of the Traveling Pants then it is A-List, The Clique, girls-behaving badly,” Nelson said of the series.

Even so, Carter said, fan mail tends to focus on the romance. In particular, she said, readers single out a section in book five, Out of Sight, Out of Time, in which protagonist Cammie has amnesia and goes missing. After being found, she asks love interest Zach where he went when he was looking for her. His reply: “ ‘Crazy.’ His voice was a whisper against my skin. ‘I went crazy.’ ”

“I get more letters about that one line than the rest of the series combined,” Carter said with a laugh.

Closing the File

Finishing the Gallagher Girls series has been “bittersweet,” the author said. “I have very mixed emotions, as you might imagine. On the one hand, very excited to finish the series and ride it out to its conclusion, very excited to bring the conclusion to the fans, very excited to see my characters ride off into the sunset. On the other hand, I am definitely going to miss that world. I am going to miss those characters.”

She admitted that she has not quite realized the series is over and still finds herself thinking up good titles for future Gallagher Girls installments. But, she added, meeting with fans on the tour will make it feel more final. “I am hoping that we have a lot of girls dress up like Gallagher Girls, come in their Gallagher Girls t-shirts and other paraphernalia, and really make it a big event.”

And what should readers expect from the final book? Carter hopes the conclusion answers all the questions readers had over the years. “I don’t necessarily owe readers a happy ending, but I owe them a satisfying ending,” she said.

While getting ready for her tour, the bestselling author is already embarking on a new mission: Scholastic will launch her Embassy Row trilogy in 2015; editorial director David Levithan will edit. In the new series, a girl goes to live with her grandfather – the most powerful ambassador in the world – in a fictional European country, and begins to piece together her the mystery behind her mother’s murder. “As she does so, she realizes that the plot goes deeper and deeper and deeper, and she get embroiled in a very big, very scary international conspiracy,” Carter said. “It’s kind of the Gallagher Girls but without the training.”

Carter said she is trying to plan the series a bit more tightly then she did with her initial YA venture. At the same time, she said, she thinks she does best when she has to write herself out of a corner. “If I am not surprised, how can I have faith that my reader is going to be surprised?” she said. “I like it when my books surprise me, and I hope that never stops happening.”