This isn't about the demise of the digital music business - that's thriving. It's about the demise of the big players in the industry. The power has shifted to the online retailers. Not really a surprise.
The book publishers have been at the mercy of their retailers for a long time. The thing that is worrisome for them is whether they can survive the battle between brick and mortar distributors and retailers (whom they are currently dependent on) and the online retailers (whom they will be dependent on).
The effect the article is talking about is already happening with Amazon and publishers. But I don't think things will get as bad as the music industry. Books' big advantage is that music went through digitization first. There were many lessons learned. So I think books will wind up in a better spot that music.