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