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Sudoku/Methods/Advanced/Simultaneous Bivalue Nishio/Sudoku Addicts Workbook
Paul Stevens in the Sudoku Addict's Workbook (2008), has an introduction for each of his 150 puzzles, describing difficulty and techniques that might need to be used. His answers also give some specifics about critical strategies. This is far above every other Sudoku book that I've found. His descriptions of strategies is thorough and reasonably clear. (It could still use improvement, and he uses punk sequential notation with tiny numbers written in a difficult-to-read script font.)
He introduces the last five puzzles with this:
- Prepare to enter the twilight zone! The final "fearsome five" puzzles are among the toughest you'll find anywhere (though still solvable using the techniques we've seen so far). They're all "project" sudokis, so take breaks and expect to solve them over periods of days rather than hours.
I have developed a "Swiss Army Knife" solving technique. Or we could call it a Gordian Knot Cutter. I start with "double dotting" (I use dots rather than numbers) and go as far with that as works, then add in triples -- often I see box triples immediately because of this -- and then all the remaining candidates. I once caught a hidden quad at this point! And then, looking at candidate positions, if I don't see some advanced strategy, I pull out Simultaneous Bivalue Nishio (SBN). I worked the puzzles in ink in the book, using distinctive additions to the candidate dots. Two I solved, two I haven't solved yet (it may have been a year ago that I did those puzzles), I'll look at them, and, in one, I made some kind of mistake, which, in ink, creates a mess. But I'll work on that puzzle too. Here they are, red links aren't done yet: