Apparently there's some controversy about this, based on the idea that more and more book sales means more and more intelligent players which makes the games tougher. In a n internet Q/A Miller had said that he doesn't think it's true that book distribution makes the games tougher. Then some sort of controversy about that arose at 2+2.
Miller's right. The toughness of poker games is pretty much a self-controlling characteristic. It has nothing to do with the size or quality of the literature.
Poker doesn't exist without cronic losers. Winning and losing doesn't depend on fixed levels of "skill", it depends on relative skills between players. On average, good players will win against bad players. But good players will lose ag ainst very good players on average.
More books might well make more good players, but will also make more very good players. Books don't make poker more or less profitable. Opponents do that.
What's been happening recently is that poker underwent a boom in popularity as a cultural fad. That boom brought a boom in book sales. The books didn't make the game more popular, but the popularity of the game caused the books.
At the very beginning of the boom there were many very bad players and it wasn't difficult for a marginal player to find a game he could win at. But, the early learning curve is quick in poker, and those very bad players quickly became just bad, and marginal players didn't win as easily, they had to start paying more attention to game selection. So, in some sense poker might have become a little tougher because of a slight increase in game selection difficulty. But certainly not because of any books.
The cultural fad is starting to fad slightly and there may be slightly fewer new players every year in the near future. I'm not sure that's going to be as significant as some people might think though.
But there will always be beatable games. Because if the games aren't beatable they dry up, with the better players finding greener pastures, leaving the weak playrs to find another game, making that game easier.
The total number of games might ebb and flow, but the ebbs come from tough games drying up and blowing away. The games left after that ebb are always going to be soft games. Winning players won't play anywhere else.