Buying a book or a piece of music should be regarded as a license to enjoy it on any platform. Sadly, the anachronistic conventions of bookselling and copyright law lag the technology. Thus you’ve violated the publishing company’s legal right to control the distribution of its intellectual property, but you’ve done no harm or so little as to meet my threshold of acceptability.
I think this doesn't go far enough. Really, piracy (or 'free riding' generally) is only wrong if you would otherwise have paid for it. If you'd sooner go without than shell out the demanded price for a non-rival good, then you may as well free ride -- it's better for some (namely, you) and worse for no-one. Don't get me wrong: it's important that those who find the good worth the price do pay, so that there's incentive to provide such goods in the first place. It's merely those who wouldn't pay anyway who can, on this view, permissibly free-ride.
Admittedly, it may not be a good idea to publicize this principle too much, since it might easily be abused by the unscrupulous. It creates an incentive for self-deception, as a wannabe pirate who really does find some good worth the price might try to convince himself that he doesn't want it that much, in a (wrongful) attempt to excuse free-riding. But one cannot expect moral principles to be proof against knavery. Sure, the unscrupulous might make insincere and inappropriate appeal to the principle, but that doesn't mean it isn't true. I'd expect that someone sincerely concerned to adhere to this moral principle would "play it safe" by only pirating in cases where it's clear that they wouldn't buy the good in any case. (If the content-producers have an online "tip jar", they might even contribute whatever lesser sum the good is worth to them.)
What do you think?