This is an excellent book, with lots of difficult puzzles. One really nice thing about this series as a whole is that it provides you a clear path of improvement. If you are using the Mensa Guide to Solving Sudoku: Hundreds of Puzzles Plus Techniques to Help You Crack Them All or
Mastering Sudoku Week by Week: 52 Steps to Becoming a Sudoku Wizard (which is actually my favorite), this is a great series to have alongside.
One problem I have with most Sudoku reviews is that they are not specific about what techniques are and are not required, so I will attempt to list here all the ones I've found required in the book and in fact the whole series. Book 1 in the series started relatively easy - about as hard as a typical "hard" puzzle in other books, requiring no more than assiduous application of basic Sudoku techniques (which includes pairs, triples, and quads). Book 2 starts basically the way book 1 left off, but about halfway through adds a number of more advanced techniques.
Book 3 (this book) picks up right where book 2 left off, with an X-wing and XY-wing in the first two puzzles. Needless to say that it's starting off quite well! I will update this review as I get farther through the book.
You will need to know the following types of techniques:
* Elementary techniques (cross-hatching, single cell, single candidate)
* Locked candidate elimination
* Naked pairs, triples, and quads
* X-wings: LOTS of them, some puzzles requiring two different X-wing patterns
* Non-unique (or "Gordonian") rectangles, including some that are beyond the simple ones (see one of the two books above for more explanation)
* XY-wings and XYZ-wings
EDITED: I'm now just past 50 puzzles in the book, and these puzzles included my first legitimate jellyfish (order 4 fishy pattern) and several that require multiple X-wings and non-unique rectangles. Round about puzzle 50, they start requiring multiple advanced techniques per puzzle.
The book is spiral bound, which is very nice. I really like having the cover continue over the binding. Although it would be slightly easier to have it lay flat without the cover there, the space makes a perfect place to store your pencil when you are required to do non-Sudoku activities. Some might dislike the concept of paying for Sudoku books at all - countering that puzzles are available online for free - but I think there's great benefit in having a book you can take with you.
Overall, a great series of books. Start by buying the one that applies to you and get going improving your Sudoku!