Question: Which successful WB Television Network series includes a horror trilogy published as a book in 2001?
Context: The first season of "Supernatural", an American fantasy horror television series created by Eric Kripke, premiered on September 13, 2005, and concluded on May 4, 2006 after 22 episodes.  It focuses on brothers Sam and Dean Winchester as they track down their father, John, who is on the trail of the demon who killed their mother and Sam's girlfriend.  During their travels, they use their father's journal to help them carry on the family business—saving people and hunting supernatural creatures.  Jared Padalecki and Jensen Ackles star as Sam and Dean, with Jeffrey Dean Morgan recurring as their father, John, and Nicki Aycox as the demonic Meg Masters.  This is the only season to air on The WB Television Network, with all subsequent seasons airing on The CW Television Network, a joint venture of The WB and UPN. The CW Television Network (commonly referred to as just The CW) is an American English-language broadcast television network that is operated by The CW Network, LLC, a limited liability joint venture between CBS Corporation, the former owners of United Paramount Network (UPN), and Warner Bros.  Entertainment, a division of Time Warner, former majority owner of The WB Television Network.  The "CW" name is an abbreviation derived from the first letters of the names of its two parent corporations (CBS and Warner Bros.). Kids' WB was an American children's programming block that originally aired on The WB Television Network from September 9, 1995 to September 16, 2006.  On September 23, 2006, the block moved to The CW, which was created by CBS Corporation and Time Warner as a replacement for both The WB and UPN.  The Kids' WB television block was discontinued on May 17, 2008, with its Saturday morning programming slot being sold to 4Kids Entertainment and replaced by successor block The CW4Kids. The third season of the animated television series "Teen Titans", based on the DC comics series of the same time by Bob Haney and Bruno Premiani, originally aired on Cartoon Network in the United States.  Developed by television writer David Slack, the series was produced by DC Entertainment and Warner Bros.  Animation.  Sander Schwartz was tagged as the executive producer for the series.  This marks the last season of Teen Titans being aired on The WB Television Network, from September 2005 to December 2005, skipping insignificant episodes.  It also became the only season that the The CW did not re-air during the 2007–08 U.S network television season, as the first two seasons of the series only re-aired on Kids' WB. Christy Jenkins is a fictional character from the American television supernatural drama "Charmed", which aired on The WB Television Network (The WB) from 1998 to 2006.  The character was created by executive producer Brad Kern and was portrayed by actress Marnette Patterson.  Developed in response to the WB's request for a new character, Christy was originally planned to expand the show in a new direction for a possible ninth season or spin-off.  It was later confirmed that all future plans for the show were cancelled following the WB's closure to launch The CW. This is a list of stations which were affiliated with The WB Television Network in the United States at the time of the network's closure.  The WB shut down September 17, 2006.  Former affiliates of The WB became affiliates of The CW, My Network TV, or revert to independent status.  Some WB affiliates dropped WB programming on September 5, 2006 in favor of MyNetworkTV. The series received critical and popular acclaim, frequently being listed as one of the greatest TV shows of all time, and usually reached between four and six million viewers on original airings.  Although such ratings are lower than successful shows on the "big four" networks (ABC, CBS, NBC, and Fox), they were a success for the relatively new and smaller WB Television Network. Safe Harbor is an American television drama series that aired on The WB Television Network from September 20, 1999 to November 28, 1999.  The series was created and executive produced by Brenda Hampton, who at the time was best known for work on the fellow WB series "7th Heaven", the series was paired with "7th Heaven" on the network's Monday night lineup.  Despite "7th Heaven" being the No. 1 show on The WB during the 1999-2000 season, "Safe Harbor" was unable to hold a solid audience after "7th Heaven" and was canceled after ten episodes and one season with the show moving to Sunday nights where the last two episodes aired. Unseen is a horror trilogy, written by Nancy Holder and Jeff Mariotte and published in 2001 by Pocket Books.  It is part of the Buffy the Vampire Slayer series. The WB Television Network (commonly shortened to The WB and short for Warner Bros.) was an American television network that was first launched on broadcast television on January 11, 1995, as a joint venture between the Warner Bros.  Entertainment division of Time Warner and the Tribune Broadcasting subsidiary of the Tribune Company, with the former acting as controlling partner.  The network principally aired programs targeting teenagers and young adults between the ages of 13 and 34, with the exception of its weekday daytime and Saturday morning program block, Kids' WB, which was geared toward children ages 7 to 12.
Answer: Buffy the Vampire Slayer