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 don't always agree with The Ethicist, but this advice is spot-motherfucking-on.
* Unfortunately, the book was Stephen King's Under the Dome, which I read. Stephen King should write The Ethicist and ask whether it's okay to write a thousand-page book with one of the most idiotic endings ever put to paper.