Actually, the film is based on a novel by Koushun Takami. This novel has also been adapted in an ongoing manga, drawn by Masayuki Taguchi that is published in the Young King magazine. So far, there are 5 tankoubon paperbacks, but an end is not yet in sight.
The manga shares the films disturbing imagery, but because of the change of medium, the portrayed scenes of violence and slaughter are even more detailed and graphic. Scenes that could not be done on film are detailed in their gory detail. The characters are explored in more detail, their backgrounds better explained than in the movie, which makes their final fates even more horrifying.
Still, the artist did not borrow the faces of the actors to draw his characters but instead chose to use his own imagination. The director for example, who was played by Takeshi Kitano, is portrayed here as a fat pig, clearly enjoying the scenes of death and horror, while stuffing himself with burgers and sodas.
I do not recommend the manga to the weak of heart, as they might find the images too disturbing. If you liked the film, or take such things lighter, you might also like this manga.
Here's a link to a Battle Royale site: