We're always being told that tie-in books and comics "don't count" — even when they're really terrific and take the series in great directions. Doctor Who's New Adventures, Star Wars' Expanded Universe... they're not "real." But which series has spawned tie-in books, comics or audio stories that are better than some "official" installments?
Please include a picture, book cover, comics panel or other representation of your superb tie-in media. And please mention the title of the book or other media, and why it's better than some of the canonical movies or TV episodes in the series. Thanks!