(I know, I know! I haven't been able to contact the author)

Does anyone here have a copy of "Mastering Sudoku" and can help me with puzzle #15? I can type it in if necessary. The exercise is on pairs, and I just don't understand the concept. Sometimes there are only two candidates, sometimes more, and there seem to be a variety of things you can do with the information gained. I need a reference with many, many examples, not yet another treatise with a few ill-defined rules.

Specifically, in the partially solved puzzle #15 on page 71, there's supposed to be a pair that "leads to a long chain of solved squares". I find a couple of redundant pairs, and another pair that eliminates a 2 from cell r9 c1, but I can't see how that helps.

Thanks