Question: What is the book series, written by Cecily von Ziegesar, that the character Eric van der Woodsen appears in?

full_context: Document 1: Erekosë: Erekosë is a character in author Michael Moorcock's fantasy novel "The Eternal Champion". He is an incarnation of Moorcock's character the Eternal Champion, and is unique among those incarnations in that he can remember his previous lives, something that frequently causes him great distress.
Document 2: Georgina Sparks: Georgina Sparks is a fictional character in the "Gossip Girl" novel series and a recurring character on the television series of the same name, in which she is portrayed by Michelle Trachtenberg. While the novel series portrays her as a harmless, friendly drug-addicted young girl, the television adaption portrays her as a cruel, manipulative young socialite who creates havoc throughout the show. Originally conceived as the series' primary antagonist, by the final season Georgina came into the good graces of the Upper East Side - even becoming a member of their inner circle. She is notable for her manipulative machinations and sudden returns throughout the series, and for appearing in all the finales, and most recently the latter half of the fifth season in light of cast members Taylor Momsen and Jessica Szohr departing. In the sixth and final season of the show, she appears in 8 of 10 episodes.
Document 3: Jenny Humphrey: Jennifer Tallulah "Jenny" Humphrey is one of the characters in both the "Gossip Girl" and "The It Girl" series of novels by Cecily von Ziegesar. She is portrayed by Taylor Momsen in the "Gossip Girl" television adaptation on The CW.
Document 4: Dan Humphrey: Daniel Randolph "Dan" Humphrey is a fictional character in the best selling "Gossip Girl" series of novels. He is one of the central male characters in the television adaptation of the same name, where he is portrayed by Penn Badgley. Dan Humphrey is the son of Rufus Humphrey and has a younger sister, Jenny, while his mother remains absent for the majority of the series. Dan and his family live in Brooklyn, New York, the alternative of the old-moneyed and conservative Upper East Side. He attends St. Jude's Preparatory School for Boys on the West Side as a scholarship student. His life changed dramatically when his father, Rufus Humphrey, married the wealthy Lily van der Woodsen, moving the family to the Upper East Side.
Document 5: The Pepperwood Chronicles: The Pepperwood Chronicles is a fictional book about "a hard-boiled Chicago cop turned New Orleans detective" named Julius Pepperwood. The book was written by the fictional character Nick Miller from the American sitcom television series New Girl. Nick says a major theme in the story is about "finding the alligator within."
Document 6: Eric van der Woodsen: Eric van der Woodsen (spelled Erik in the novels) is a fictional character from the book series "Gossip Girl" and the television series of the same name, in which he is portrayed by Connor Paolo.
Document 7: Gossip Girl: Gossip Girl is an American teen drama television series based on the book series of the same name written by Cecily von Ziegesar. The series, created by Josh Schwartz and Stephanie Savage, originally ran on The CW for six seasons from September 19, 2007, to December 17, 2012. Narrated by the omniscient blogger "Gossip Girl", voiced by Kristen Bell, the series revolves around the fictional lives of upper-class adolescents living in Manhattan's Upper East Side.
Document 8: Eric Northman: Eric Northman is a fictional character in "The Southern Vampire Mysteries," a series of thirteen books written by "New York Times" bestselling author Charlaine Harris. He is a vampire, slightly over one thousand years old, and is first introduced in the first novel, "Dead Until Dark" and appears in all subsequent novels. Since the book series is told from the first person perspective of Sookie Stackhouse, what readers perceive of his character is influenced by what Sookie comprehends. HBO's television series "True Blood" is based on this book series and the character of Eric Northman is portrayed somewhat differently. A list of "True Blood" characters has a detailed description of Eric's character from the TV show.
Document 9: The It Girl (novel series): The It Girl is a series of novels created by bestselling novelist Cecily von Ziegesar. The series is ghostwritten from the original idea by von Ziegesar. The series, aimed toward young adults, is a spin-off from the bestselling "Gossip Girl" series.
Document 10: List of Gossip Girl episodes: "Gossip Girl" is a teen drama which premiered on September 19, 2007 in the United States on The CW. "Gossip Girl" is based on the young-adult book series of the same name written by Cecily von Ziegesar, and was developed for television by "The O.C." creators Josh Schwartz and Stephanie Savage. The series follows the lives of the young, wealthy, and social elite residing in the Upper East Side of Manhattan, and is narrated by an unseen and seemingly omniscient character, "Gossip Girl", whose blog is widely read among the characters.
Document 11: Gossip Girl (season 1): The first season of "Gossip Girl", an American teen drama based upon the book series by Cecily von Ziegesar. Developed for television by Josh Schwartz and Stephanie Savage. Airing on The CW from September 19, 2007 till May 19, 2008 with 18 episodes. It tells the story of Serena van der Woodsen's return to Manhattan's Upper East Side in New York City following her mysterious disappearance to boarding school in Connecticut, while being watched by gossip website Gossip Girl.
Document 12: Elijah Connor: Elijah Connor (born July 20, 1988 in Detroit, Michigan) is an American pop singer, songwriter and actor. He is best known for his role on Donald Trump's reality television series "The Ultimate Merger". He landed a minor role in the movie "Sparkle" starring Whitney Houston, Mike Epps, and Jordin Sparks. He played himself in the NBC/Oxygen drama series "Player Gets Played".
Document 13: Serena van der Woodsen: Serena Celia van der Woodsen is a main character in the "Gossip Girl" novel series and its TV adaptation, in which she is portrayed by Blake Lively. Serena is featured on the blog of the series' mysterious narrator, "Gossip Girl." Serena is Blair Waldorf's best friend and is a character that appears to easily get whatever she wants, because of her good looks, energy and charisma. She is also known as the "It Girl" in the series.
Document 14: List of Gossip Girl characters: The following is a list of characters for The CW teen television drama series, "Gossip Girl". The show is based on the popular book series of the same name, written by author Cecily von Ziegesar.
Document 15: Gossip Girl (novel series): Gossip Girl is an American young adult novel series written by Cecily von Ziegesar and published by Little, Brown and Company, a subsidiary of the Hachette Group. The series revolves around the lives and romances of the privileged teenagers at the Constance Billard School for Girls, an elite private school in New York City's Upper East Side. The books primarily focus on best friends Blair Waldorf and Serena van der Woodsen, whose experiences are among those chronicled by the eponymous gossip blogger. The novel series is based on von Ziegesar's experiences at Nightingale-Bamford School and on what she heard from friends.
ground_truth: Gossip Girl