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You don't use fanfiction to read published books. Fanfiction is a 'spinoff' of sorts to works already produced, whether books, movie, television, stage, or sometimes the lives of real life celebrities. Occasionally, fanfiction is written in multiple chapters, even reaching the length of a novel. You can read it chapter by chapter as you would a published novel. Or, if you're looking for something to accompany a book you already love, you can read fanfiction to gain other characters' points of view, alternate endings, or even alternate worlds. But fanfiction isn't something that assists you with reading a book.