That's really down to your personal ethics and how much you are willing to tolerate. As you're not distributing it, the chance of someone coming after you for

That's really down to your personal ethics and how much you are willing to tolerate. As you're not distributing it, the chance of someone coming after you for downloading a non-legal copy is almost non-existent, and pretty much 0 if you've taken some basic precautions. It sounds like you've answered your own question there; that as you'd read the majority of the book, that there's no real loss to you finishing it for free.