Question: What bestselling book that was a sequel to the 1937 novel the Hobbit is also being adapted by the Tolkien Ensemble?

Context articles: An Evening in Rivendell
The Tolkien Ensemble
Smaug
Thorin Oakenshield
Leaving Rivendell
At Dawn in Rivendell
Bilbo Baggins
The Hobbit: An Unexpected Journey
The Hobbit (film series)
The Lord of the Rings

An Evening in Rivendell is the first album by the Danish group The Tolkien Ensemble.
 It features songs composed to the lyrics found in J. R. R. Tolkien's "The Lord of the Rings" and forms the first part of what was to become a complete musical interpretation of all lyrics in the book.
The Tolkien Ensemble (founded in 1995) is a Danish ensemble which aims to create "the world's first complete musical interpretation of the poems and songs from "The Lord of the Rings"".
 They published four CDs from 1997 to 2005, in which all the poems and songs of "The Lord of the Rings" are set to music.
 The project was approved by both the Tolkien family and HarperCollins Publishers.
 Queen Margrethe II of Denmark gave permission to use her illustrations in the CD layout.
Smaug ( ) is a fictional character and the primary antagonist in J. R. R. Tolkien's 1937 novel "The Hobbit".
 He is a powerful, fearsome dragon who invaded the Dwarf kingdom of Erebor 150 years prior to the events described in the novel.
 A group of 13 Dwarves mounted a quest to take the kingdom back, aided by the wizard Gandalf and the Hobbit Bilbo Baggins.
 Smaug is described as "a most specially greedy, strong and wicked wyrm".
Thorin II Oakenshield, son of Thráin, son of Thrór, King under the Mountain is a fictional character in J. R. R. Tolkien's 1937 novel "The Hobbit".
 Thorin is the leader of the Company of Dwarves who aim to reclaim the Lonely Mountain from Smaug the dragon.
 He is the son of Thráin II, grandson of Thrór, and becomes King of Durin's Folk during their exile from Erebor.
 Thorin's background is further elaborated in Appendix A of Tolkien's 1955 novel "The Return of the King".
Leaving Rivendell is the fourth album by the Danish group The Tolkien Ensemble.
 It features songs composed to the lyrics found in "The Lord of the Rings" by J. R. R. Tolkien and forms the end part of what was to become a complete musical interpretation of all lyrics in the book.
At Dawn in Rivendell is the third album by the Danish group the Tolkien Ensemble.
 It features songs composed to the lyrics found in J. R. R. Tolkien's "The Lord of the Rings" and forms the third part of what was to become a complete musical interpretation of all poems and songs in the book.
 It is also the first album to feature actor Christopher Lee reciting the spoken poems - something that brought the ensemble to the attention of many internationally.
Bilbo Baggins is the title character and protagonist of J. R. R. Tolkien's 1937 novel "The Hobbit", as well as a supporting character in "The Lord of the Rings".
 In Tolkien's narrative conceit, in which all the writings of Middle-earth are translations from the fictitious volume of "The Red Book of Westmarch", Bilbo is the author of "The Hobbit" and translator of various "works from the elvish" (as mentioned in the end of "The Return of the King").
The Hobbit: An Unexpected Journey is a 2012 epic high fantasy adventure film directed by Peter Jackson.
 It is the first installment in a three-part film adaptation based on the 1937 novel "The Hobbit" by J. R. R. Tolkien.
 It is followed by "" (2013) and "" (2014), and together they act as a prequel to Jackson's "The Lord of the Rings" film trilogy.
 The film's screenplay was written by Peter Jackson, his longtime collaborators Fran Walsh and Philippa Boyens, and Guillermo del Toro, who was originally chosen to direct the film before leaving the project in 2010.
The Hobbit is a film series consisting of three high fantasy adventure films directed by Peter Jackson.
 They are based on the 1937 novel "The Hobbit" by J. R. R. Tolkien, with large portions of the trilogy inspired by the appendices to "The Return of the King", which expand on the story told in "The Hobbit", as well as new material and characters written especially for the films.
 Together they act as a prequel to Jackson's "The Lord of the Rings" film trilogy.
 The films are subtitled "" (2012), "" (2013), and "" (2014).
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.