Book excerpt: Sudoku designers the world over will weep and gnash their teeth at the revelations in this comprehensive guide to cracking the addictive puzzles--but solvers will find it absolutely invaluable as they seek to improve their skills. Even experts don't know all these tricks: hidden pairs, naked pairs, X-wings, jellyfish, squirmbag, bivalue and bilocation graphs, and chains, plus the exclusive Gordonian logic methods that turn the toughest puzzles into a breeze. There are hundreds of sudoku to practice on. A special addition is a reprint of the very first sudoku ever published "