If you can't get a book legally or conveniently, there isn't really any sales to be "lost" because you wouldn't buy it. There is no real injury if you can't acquire something otherwise. You're not stealing a book from a store to cause a loss.

So, it's either do without or find a way to get it. And then, you might make an extra effort to acquire it if it's really good and encourage others to find it too. Not as a rationalization but as a human habit: pirating some content, within reason, leads to increased sales overall rather than a decrease.