And trapped between the commodity supplier fixing the price, and the state bringing up the costs, its a solid recipe for killing off a business. Which it has done in this case.
So the more interesting question from an economics perspective is whether or not capturing the marginal GDP of this business is a better or worse for the people. Sure the bookstore goes poof, and folks can't get cheap used books any more. The business will be replaced by something that can afford both the higher labor rate, like an upscale coffee shop or maybe a designer boutique.
But the number of bookstores goes down by one, and the volumes available are not available in the library. So the community has lost something too.
Nominally the markets are there to express what is a good application of capital by meeting demand. Should it go to running a bookstore or running a coffee roaster or a blog aggregator Etc. But the artificial floor of minimum wage creates artificial barriers to applying capital. You can see in this post that the floor of $10 allows the bookstore to exist, the floor of $15 does not. So here is a fiscal policy which is going limit choices.
And a really interesting effect is that everyone is going to raise prices to cover that expense, they have too, and so all of those folks who were complaining that they can't afford to eat at the 'upscale' bars and bistros in California its just going to get worse for them, and it won't be the Googlers or Twitterers or Pinsters causing that upswing, it will be the very government they elected to serve them doing it to them.
I find the economics of such things very interesting in the side effects as much as the intended effects.