The vast majority of sudoku puzzles that are labeled hard " "expert " "challenger " are not really all that difficult. & in a surprising number of cases they are downright easy. In the " Absolutely Nasty Sudoku" books you will find puzzles truly worthy of the label "hard." The puzzles increase in difficulty throughout the four books in the series. It ranges from basic & intermediate puzzles & techniques at Level No. 1 to the most advanced techniques at Level No. 4 along with puzzles that require multiple advanced techniques each time."