What I want to say about The Hidden Logic of Sudoku (second edition) is that the writer has pushed the underlying logic of xy-chains to its limits, he has introduced xyt-chains and shown that they are a very natural and powerful generalisation of xychains. He has also defined another independent generalisation, the xyz-chains, and combined it with the previous one to get the xyzt-chains.
If you continue your reading, you will also find the hidden and "3D" generalisations of these patterns.
Andrya wrote:But can you tell me that does this promo exist still?
The particular promotion announced above is no longer valid.
Lulu promotions are decided by Lulu alone. They often have promotions on their website (upto 20 or 25% discount); but recently, they introduced a second type of promotion (upto 30%) with short validity, for which they send each time a new promotion code to the author. But AFAIK they have sent authors no information about this promotion policy. To me, both types seem to appear at random times. (I conjecture that randomness is part of their policy, so that people visit their site more often.)
Andrya wrote:I think that the price offered by you a little bit increased as compared with other promos...!
I don't understand. How can the price (offered by Lulu, not me) be increased by the promo?