Question: Denethor appears in a fictional novel that began as a sequel to what novel?

Context articles: Denethor
John Sampson (character)
Coco and Igor
The Lord of the Rings
Angel of Music, or The Private Life of Giselle
Them: A Novel
Ben Singkol
Begone, Demons
Nightmare Academy
A Woman in Amber

Denethor II, son of Ecthelion II, is a fictional character in J. R. R. Tolkien's "The Return of the King", which is the third and final part of his novel "The Lord of the Rings".
 He is the 26th and final Ruling Steward of Gondor.
John Sampson is a fictional character in the mystery fictional novel series, "Alex Cross (novel series)" and is one of the main characters.
Coco and Igor is a 2002 novel by Chris Greenhalgh.
 Set mainly in Paris in 1920, the fictional novel is based on a possible real-life affair between Coco Chanel and Igor Stravinsky.
The Lord of the Rings is an epic high fantasy novel written by English author and scholar J. R. R. Tolkien.
 The story began as a sequel to Tolkien's 1937 fantasy novel "The Hobbit", but eventually developed into a much larger work.
 Written in stages between 1937 and 1949, "The Lord of the Rings" is one of the best-selling novels ever written, with over 150 million copies sold.
"Angel of Music, or The Private Life of Giselle" (2007) is a fictional novel by Russian author Maria Andrianova.
 It is a sequel to original Gaston Leroux's novel "The Phantom of the Opera" (1910).
 This book is the first officially published Russian fanfiction work on the Phantom of the Opera theme and it is illustrated by the author, who is a professional artist.
Them: A Novel is a 2007 debut fictional novel by Nathan McCall.
Ben Singkol is a 2001 novel written by Filipino National Artist F. Sionil José.
 It is about Benjamin "Ben" Singkol, who is described as “perhaps the most interesting character” created by the author.
 Based on José's novel, Singkol is a renowned novelist who wrote the book entitled ""Pain"", an autobiography written during the Japanese occupation of the Philippines.
 Through the fictional novel Singkol recalled the hardships experienced by the Filipinos during the occupation.
 Singkol was described to be a coward, a ""supot"" or an uncircumcised man who did not only run away from such a “ritual of manhood” but also evaded his “foxhole in Bataan when the Japanese soldiers were closing in”.
 Singkol was a “runner” or “evader” throughout much of his lifetime, while being haunted by the “poverty of his boyhood” and of the “treachery that he may have committed” in the past.
 In 1982, Singkol began receiving letters from a Japanese named Haruko Kitamura.
Begone, Demons, also translated as Get Out You Damned, or Get Out of Here, Curse You!
 is Saddam Hussein's fourth and last novel.
 It is a fictional novel, with political metaphor.
 It is thought to have been written in anticipation of the 2003 Iraq War in 2002 or 2003.
 It was not yet published before the 2003 Invasion of Iraq.
 It was published in Japanese in 2006.
Nightmare Academy is a 2002 Christian fictional novel by Frank Peretti and the second novel in the Veritas Project series authored by Frank Peretti.
 The book was one of the ALA's young adult book picks for 2004.
A Woman in Amber: Healing the Trauma of War and Exile is a part autobiographical, part fictional novel written by Agate Nesaule.
 The first half of the novel describes Nesaule’s experiences of exile from Latvia imposed by the invading Soviet army, and her family’s emigration to the United States in 1950.