Why do blackjack books seem to disagree on basic strategy?
The correct basic strategy depends on the exact details of the game to which it applies. The number of decks, and the various rule changes from casino to casino make a difference in the correct strategy. Most authors when writing about blackjack have chosen to describe a basic strategy which applies to the most common games, often without explaining for which exact game their charts are intended. Some authors explain which game their charts assume, but this may be a different game than other authors chose. And, unfortunately, some author’s charts are simply incorrect for any game.