The Bleach bootleg was written by Kubo and it is not a fake in any way. Why it is called a bootleg however, has not been explained. Possibly, it is a joke concerning the nature of the content. The Bleach Bootleg consists entirely of Shinigami material. The second half of the book is even made to appear like a copy of the Seireitei Communication. So the "bootleg" aspect of it is that "The Official Bleach Bootleg" book was illegally smuggled out of Soul Society for consumption by humans.