The Hobbit is a prequel to The Lord of the Rings written by J.R.R. Tolkien, it is based on Bilbo Baggins journey to the Misty Mountains and how he came across the one ring. Peter Jackson, whom directed The Lord of the Rings Trilogy, adapted the whole book into a three part film, however the film was not entirely from the book despite the whole book being adapted. Peter Jackson found himself adding more into The Hobbit so that he could make the film into three parts, each being almost two hours long. In The Lord of the Rings Jackson is known for having to cut out parts from the book in the film adaptation in order to make the films short enough to fit the entire story and also having a reasonable viewing time in box office.
The Lord of the Rings was so widely received and therefore they already had the fan base for The Hobbit if it were adapted, it was already a much loved book albeit it was smaller then all three of The Lord of the Rings books. Due to The Hobbit's book size it was obvious that it would need to be added to and adapted to be made into a trilogy, things would need to be added in order for it to be a lengthy film in box office and for it to also be made into three films.
By adapting the book and adapting it further by adding extras into the films in order to make it into a trilogy, Jackson effectively changes the story from doing this, making it a different one to the books. Jackson studied the works of J.R.R. Tolkien in order to adapt this film correctly and in they way of his texts, while writing the script they found themselves constantly adapting it to fit to the flow of the book and to not interrupt the book and its story, the adaptation took time and delayed filming.
The film adaptation is great without a doubt, it is adapted well and manages to use the aspects from the book with extras scenes, making the film flow and work great as a flowing storyline. Jackson managed to pull of a film that was only really half there in the books and managed to create the other half from studying writings and J.R.R. Tolkien's notes on the books. Overall he creates a great film adapted from the book and manages to keep the film in check with the books story and theme.