Book excerpt: Puzzlers ready to live dangerously and push beyond their mental comfort zone should steel themselves for the second book in the Times' toughest Killer series yet Su doku fanatics should prepare themselves for the extreme mental challenge of 120 Deadly-level Killer puzzles, including, for the first time ever, 20 Extra Deadly, to really test one's limits: this is the only series offering such a torturous collection. Not for the fainthearted, these puzzles use the same 9x9 grid as Su Doku, with an added mathematical challenge. The aim is not only to complete every row, column, and cube so that it contains the digits 1–9, it is also necessary to ensure that the outlined cubes add up to the same number as well. Unlike with previous books, there is no chance to ease in with Easy, or even Tough puzzles: this book is the ultimate Killer Su Doku—completely deadly.