Sudoku is addictive, and, like everything else, the more you do it, the better you get at it. Unlike everything else, however, with Su Doku, as you crack one level of difficulty, you need to have a more difficult set of problems to go at. This set of three books is dexcribed as "extreme", and there's no doubt that these are the ultimate Su Doku puzzles in terms of difficulty. I exclude those puzzles where you need to guess something, and then work it round to see if the guess was right or not. I don't really think that they can be counted as proper logic puzzles, because they involve guesswork. If you're into that, then some of the books from the Daily Telegraph series - "Fireside" and "Christmas" Su Doku - might suit you. They are not as logically difficult, but frequently require some guesswork and then checking. I do not enjoy those - guesswork doesn't seem to me to be a good way to solve puzzles (unless you're a computer, in which case it's a very good way indeed, but you need to be very quick!).
Wayne Gould seems to have understood exactly what Su Doku is all about very early on, and worked out a way of controlling the difficulty of his puzzles perfectly. There is NO guesswork (sometimes euphemistically referred to as "Forcing Chains" or "Nishio" - names invented, I suggest, to add plausibility to guessing!). But, make no mistake, these puzzles are hard. X-wings are often in as a matter of course. The second half of the book contains the occasional Swordfish, and these are REALLY difficult to spot - excellent! There is even, occasionally, a hidden quad, which are beautiful when you find them, but not very common.
All in all, if you've reached the point where most Su Doku puzzles in newspapers are fairly easy (even the "Super Fiendish"), and you don't enjoy the "let's try this and see if it works" approach, then this set of books is for you. You won't be racing through these! But be warned, they are addictive!