What bestselling book that was a sequel to the 1937 novel the Hobbit is also being adapted by the Tolkien Ensemble?
The Tolkien Ensemble: 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"". The Lord of the Rings:  The story began as a sequel to Tolkien's 1937 fantasy novel "The Hobbit", but eventually developed into a much larger work.
The Lord of the Rings