I bought the puzzle book for my mom who had complained that another book she purchased that was rated 'difficult' was too easy. Well this book was definitely challenging for her. She also liked the fact that it was spiral bound so that she can turn the pages all the way over.

This series is the best sudoku series available for intermediate and advanced players. The level 4 is close to level 3, but there are more puzzles with difficult linked chains and other advanced techniques. If you have trouble with these you need to buy the Mensa guide to solving sudoku.

I enjoy this book, but strongly suggest not buying it unless you have exhausted/outgrown the other Sudoku books. This is the hardest book I have found by far. I didn't work through levels 1 - 3 (probably should have). The next hardest book I have is "Even Higher Sudoku". The latter might be good to try before moving on to this book.

This is the only Sudoku book I've found where I can't do over half of the puzzles without notes. So if you want to tax your brain a bit and know about Gordian Polygons then this is the sudoku book to buy. Makes the "black belt" and "ultimate ninja" books look easy by comparison. Still all logic and no guessing required.

As someone who searches for seriously difficult sudoku, I love this book. The puzzles are all very hard, but still require a variety of solving methods, some of which I find approachable, and others of which are truly painful, at least for me. I agree with another reviewer that it would be nice to have "speaking" solutions, that is, solutions that explain how the puzzle was solved, but solving methods vary so much by player that this is probably only a dream. There are truly no linear solution methods, which is what makes sudoku so attractive.

Compared to the many other books that claim to offer very hard sudoku, this one stands way out as telling it like it is.

I criticised the "Mensa Guide" book for its fragile binding and tiny grids, so I have to award big points for the spiral binding and large grids of this book.